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UNIVERSITY OF LEEDS
INVESTIGATION
OF
LOADED MONOPOLE ANTENNA
BY
NIKOLAOS — HRISSOVALANTIS VARDALAHOS
PROJECT SUPERVISOR: M. B. STEER
Submitted in accordance with the requirements for the degreeof
Master of Science (Eng.)in
Radio Communication and High Frequency Engineering
The University of LeedsDepartment of Electronic & Electrical Engineering
September 2000
The candidate confirms that the work submitted in his own and that appropriate credit hasbeen given where reference has been made to the work of others.
Investigation of Loaded Monopole Antenna
Abstract
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This Issue Originated By: Approved By:
N. H. VardalahosMSc Student
M. B. SteerDirector of Microwave & PhotonicsProject Supervisor
Investigation of Loaded Monopole Antennas
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N. H. VARDALAHOS 08/09/2000Page ii
AC K N O W L E D G E M E N T S
Assistance during this project has been gratefully received from Prof. M. B. Steer
(Project’s Supervisor). The author wishes to thank him for his advice and instruction
throughout the project, and the provision of various ideas to improve the antenna
design. Thanks are due to project supervisor for his continual support and patience.
The author also wishes to thank Dr. G. B. Lockhart (Assessor Supervisor) for his
assistance and patience to improve the final report.
Further thanks to Dr. H. J. Strangeways and Dr. J. R. Richardson for their
information and assistance to my research on broadband antenna design. Their
literature references helped me to overcome major problems of this project.
Acknowledgements are given also to the personnel of the departmental workshop for
their professional fabrication of the antenna elements.
The author would like also to convey his thanks to the Engineering and Physical
Sciences Research Council (EPSRC) for its postgraduate studentship award, which
supported him during this year. In addition, many thanks to all of those who offered
him the opportunity to experience the great advantages of the M.Sc. (Eng.) course in
Radio Communications and High Frequency Engineering at the University of Leeds.
At this point, author does not like to forget his parents and his friends who still
support him during his academic year in Great Britain and especially during this
difficult year.
This report is dedicated to all of them with thanks for what they have done to him.
From the bottom of my heart thank you all!
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ABST RACT
Wireless cellular telephony has become one of the most important modes of
communication. Mobile telephones have the potential to be a mandatory accessory of
our lives. The number of wireless standards (at different frequencies) increases rapidly
and makes the operation of communication devices over a broad band necessary.
This report is an M.Sc (Eng.) thesis of a project on loaded monopole antennas. It
analyses the monopoles loaded by reactive to obtain antennas broadband
characteristics.
The major objective of this project was the minimisation of the antenna losses over
the operational broad bandwidth, with bandwidth defined such that the VSWR is than
four and the antenna power gain positive.
A bandwidth ratio of 11:1 was achieved with a 0.143-m capacitively loaded monopole
of 4 mm-wire radius. The operational bandwidth of this monopole starts at 800 MHz
and ends at 4,000 MHz with a VSWR less than 3.5, a power gain nearly 5.0 dB was
achieved, and a matching network not required.
Measured results for a capacitively- loaded monopole with 3 mm conductor radius are
reported. The results are similar to those simulated. However, the finite ground of the
monopole that is used and the imperfect conditions in the laboratory increase the
reflections (losses) at the antenna input.
The conclusion of this report and the further work of the project are presented at the
end.
Key words: Quarter-wavelength antenna, broadband loaded monopole, capacitively
loaded monopole, inductor-capacitive loaded monopole, antenna properties, history of
the antennas, Maxwell’s electromagnetic equations, standing waves, reflection
coefficient ( ), Voltage Standing Wave Ratio (VSWR), matching network, coupling
networks, wireless communication standards, monopole antenna radiation resistance,
directivity, Numerical Electromagnetic Code (NEC), “Imperfect” ground, N-type and
SMA connectors.
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TT AA BB LL EE OO FF CC OO NN TT EE NN TT SS
Abbreviations and Symbols ............................................................................................................ 1
1 Introduction .............................................................................................................................. 5
1.1 Radio & Microwave Communication Systems............................................................................5
1.2 The Antenna..........................................................................................................................................6
1.3 Objectives ..............................................................................................................................................8
1.4 Organisation of this Report..............................................................................................................9
2 State of the Art ....................................................................................................................... 10
2.1 History of Electromagnetism and Antennas ..............................................................................10
2.2 Maxwell Equations ...........................................................................................................................11
2.3 Fundamental of Antennas ...............................................................................................................13
2.4 Impedance Matching And Coupling Networks .........................................................................19
2.5 Antenna Bandwidth and Wireless Communication Standards .............................................222.5.1 Second Generation Cellular Systems (2G) ...............................................................................................222.5.2 Digital Enhanced Cordless Telecommunications (DECT).....................................................................232.5.3 Bluetooth ........................................................................................................................................................232.5.4 Third Generation Cellular System (3G) ....................................................................................................242.5.5 Global Positioning System (GPS) ..............................................................................................................26
3 Monopole Antennas ............................................................................................................... 27
3.1 Fundamentals of Monopole Antenna...........................................................................................27
3.2 Types of Monopole Antenna...........................................................................................................29
3.3 Advantages And Disadvantages of Monopole Antennas........................................................36
3.4 Applications of Monopole Antennas ............................................................................................37
4 Design and Simulations of Loaded Monopole .................................................................. 39
4.1 Design Requirements and Concepts ............................................................................................39
4.2 Design Methodology ........................................................................................................................40
4.3 Design Tools.......................................................................................................................................41
4.4 Numerical Electromagnetic Code (NEC)...................................................................................434.4.1 NEC Input Commands (Cards) ...................................................................................................................44
4.5 Quarter-Wavelength Monopole Simulation...............................................................................47
4.6 Capacitively Loaded Monopole Antenna ...................................................................................544.6.1 Capacitively Loaded Monopole with 3 mm Radius of Conductor .......................................................544.6.2 Capacitively Loaded Monopole with 4 mm Radius of Conductor .......................................................61
4.7 Loaded Monopole with L-C Circuits ...........................................................................................68
4.8 Summary ..............................................................................................................................................74
5 Fabrication & Testing of Capacitively Loaded Monopole............................................. 75
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5.1 Fabrication Procedures and Concepts .......................................................................................75
5.2 Finite Ground Plane (“Imperfect Ground”) .............................................................................79
5.3 N-Type and SMA Connectors ........................................................................................................80
5.4 Antenna Measurement Concepts ..................................................................................................83
5.5 Antenna Testing and Results ..........................................................................................................84
5.6 Summary ..............................................................................................................................................89
6 Conclusions ............................................................................................................................. 90
6.1 Recommendation for Further Work .............................................................................................92
7 References ............................................................................................................................... 93
Appendix A ...................................................................................................................................... 98
Appendix B .................................................................................................................................... 100
Appendix C .................................................................................................................................... 102
Appendix D .................................................................................................................................... 104
Appendix E .................................................................................................................................... 106
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ABBREVIATIONS AND SYMBOLS
1G First Generation of mobile telephony
2D Two Dimensions plane
2G Second Generation of mobile telephony
3G Third Generation of mobile telephony
AC Alternating Current (implying sinusoidal signal)
CDMA Code Division Multiple Access
DC Direct Current
DCS
DECT initially Digital European Cordless Telecommunications
now Digital Enhanced Cordless Telecommunications
ETSI European Telecommunication Standards Institute
GA Genetic Algorithms
GPRS General Packet Radio Service
GPS Global Positioning System
GSM Global System for Mobile communication
IEC International Electrotechnical Commission
IP Internet Protocol
ITU International Telecommunication Union
LC Inductor and Capacitor
NEC Numerical Electromagnetic Code
PDC Personal Digital Cellular
RF Radio Frequency
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TDD Time Division Duplex
TDMA Time Division Multiple Access
UMTS Universal Mobile Telecommunications System
WAP Wireless Application Protocol
a wire radius of the antenna
B Magnetic flux density (Tesla)
c Velocity of light
C Closed contour
C Capacitance of capacitor
d Length of the segment of the antenna
D Largest dimension of the antenna
Ddip Directivity of the dipole antenna
Ddip,/2 Directivity of the half-wavelength dipole antenna
Dm Directivity of the monopole antenna
Dm,/4 Directivity of the quarter-wavelength monopole antenna
E Electrical field
f Frequency
G Material conductance
H Magnetic field
h height of the antenna from its base
Idip Current of the dipole antenna
Im Current of the monopole antenna
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j Notation of imaginary part of complex number
J Conduction current density (Amp/m2)
L Physical length of the antenna
L Transmission line length
Pr,dip Radiation power of the dipole antenna
Pr,m Radiation power of the monopole antenna
Prad Radiated power
Pin Input power
Q Quality factor
R General resistance
RL Loss resistance
Rr Radiation resistance
Rr,m Radiation resistance of the monopole antenna
S General closed surface
V Velocity of radio frequency energy on antenna
Vdip Voltage of the dipole antenna
Vg Voltage of the generator of the antenna
V i Incident voltage wave
Vm Voltage of the monopole antenna
Vr Reflected voltage wave
VSWR Voltage Standing Wave Ratio
XA Radiation reactance
ZA Antenna impedance
Zdip Impedance of the dipole antenna
Zdip, /2 Impedance of the half-wavelength of the dipole antenna
Zg Impedance of the generator of the antenna
Zm Impedance of the monopole antenna
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Zm,/4 Impedance of the quarter-wavelength monopole antenna
Zo Characteristic impedance of the transmission line
Propagation constant
Reflection coefficient
Permittivity
rad Radiation efficiency
Elevation angle of an antenna
Wavelength
Permeability
Electric charge density
Material conductivity
Phase shift
Magnetic flux normal to the coil
dip Radiation intensity of the dipole antenna
m Radiation intensity of the monopole antenna
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11 IINN TT RR OO DD UU CC TT II OO NN
Telecommunications are growing very rapidly, with mobile phones rapidly becoming
ubiquitous while at the same time becoming multipurpose too. Nowadays, mobile
phones are used to access Internet accounts, to listen radio, reading updated news, etc.
The number of mobile telephone connections is increasing rapidly (around 100mil.
connections worldwide in 1999) and tariffs have fallen around 40% worldwide over
the last three years. So mobile phones are everywhere, due to the falling prices, rising
quality and clever marketing [1].
1.1 RADIO & MICROWAVE COMMUNICATION SYSTEMS
The major role of a communication system is to transmit data from one side
(transmitter) to another (receiver) by electromagnetic energy travelling between the
two reference sides (Figure 1.1). Like any other communication system, a microwave
communication system uses transmitters, receivers and antennas. The same
modulation techniques used at lower frequencies are also used in the microwave
range. However, the RF part of the equipment is physically different because of the
special circuits and components that are used to implement the system.
Figure 1.1 General type of communication system
The transmitted signal (electromagnetic wave) of a communication system is always
followed by noise, which exists in free space as well as in the transmitter and receiver.
Noise always reduces the efficiency of the system and determines the boundaries of
the signal detection. The noise level of a communication system can be reduced by a
careful design of the transmitter and receiver and by increasing the level of the signal
Receiver
(Rx)
Transmitter
(Tx)Antenna 1 Antenna 2
ElectromagneticEnergy
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in order to overcome equipment and background (free space) noise. Of course, the
easiest solution of these problems is to increase the power at the source, but this
method is expensive and limited. Therefore, the communication system must be
designed as efficient as possible and the most effective method is that all the
components of the system (particular antenna and transmission lines) must be
“matched” in order to radiate (or receive) all the available energy from the generator.
1.2 THE ANTENNA
One of the most important components of any communication system, which depends
on the free space as the mobile telephone, is the antenna. Most antennas for radio
communications consist of metal wires or rods connected to the transmitter or
receiver.
In any communication system the roles of the antenna are the radiation of the
electromagnetic wave into the free space using the supplied energy of the source. The
second role of the antenna is the reception of the transmitted signal and the delivery of
the signal to the receiver.
In order for the antenna to transmit electric currents is forced to oscillate along the
wire. Energy from this oscillating charge is emitted into space in the form of
electromagnetic waves. These waves, on the receiver antenna, induce a weak electric
current in the antenna wire. The radio receiver amplifies this current. An antenna can
generally be used for reception and transmission on the same wavelength if
transmission power is not too great. The dimensions of an antenna usually depend on
the wavelength, or frequency, of the radio wave for which the antenna is designed [2].
The general types of antennas according to their performance as a function of
frequency are [3]:
• Electrical small antennas; the length of the antenna structure is less than a
wavelength , in extent (where =c/f, c is the velocity of light and f is the
frequency).
• Resonant antennas; operates well at a narrow bandwidth, specific examples the
/2 dipole and the /4 monopole antennas.
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• Aperture antennas; have a physical aperture through which electromagnetic
waves flow to the free space, specific examples are horn and reflector antennas.
• Broadband antennas; the parameters of the antenna’s properties are nearly
constant over a wide frequency range. Specific examples are the spiral and yagi
antennas.
The basic parameters of the properties of an antenna are [4]:
• Input Impedance, at the terminals of the antenna.
• Radiation Pattern, which defines the variation of the antenna radiation
according to the characteristic angles.
• Bandwidth that specifies a range of frequencies over which the properties of
the antenna are acceptable.
• Radiation Efficiency which is the ratio of the radiated power (Prad) to the input
power (Pin) of the antenna (i.e. rad = Prad/P in).
• Directivity, which is the ratio of the power flux density radiated by the actual
antenna in a given direction to the flux density radiated by an isotropic antenna
radiating the same total power.
• Far Field, which is the great distance from the origin of the antenna to the
points where the Electrical (E) and Magnetic (H) field intensities fall as 1/R and
the power density 1/ R2 (where R, distance from an observation point to an
origin in the antenna’s structure). In addition the E (electrical) and H
(magnetic) fields are transverse to the line of sight and in the ratio Z0 –medium
impedance- (where Z0 = E0x/H0y or for lossless medium Z0 = √(µ/ε)) . The far
field requires two conditions: R ≥ 2D2/λ and R >> λ, (where D is the largest
dimension of the antenna). On the other hand, some antennas are “electrically
large” i.e. D >>λ /2, so at that case the first condition implies the second [5].
• Gain, which defines the ratio of the far-field power flux density radiated by the
actual antenna in a given direction to the flux density that would be radiated by
an isotropic antenna, accepting the same power at its input port.
• Polarisation, which is the electric field vector of the radiated wave.
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1.3 OBJECTIVES
The objectives of this project are to investigate, simulate and fabricate a broadband
antenna for mobile telephony in order to improve the quality of operation of the
handset according to the market’s requirements. This broadband wire monopole
antenna can be placed on (e.g. cars, railway trains, ships, etc.).
The initial idea, at the beginning of the project, was to design a wire antenna than
could operate at least at the GSM networks at 900 MHz and 1800 MHz. The final
objective was to design a loaded monopole antenna, which will cover the most of the
common wireless protocols (GSM, DECT, BLUETOOTH, GPS and UMTS).
Another objective is to minimise the power consumption of the antenna and to
improve the transmission efficiency. The power loss is a result of mismatch between
the antenna impedance (ZA) and the characteristic impedance of the transmission line
(Zo) and of the losses, which are generated on the conductor, due to its resistance. The
minimisation of the power loss can be achieved by using a matching network between
the transmission line and the input of the antenna. Nevertheless, a good design can
limit reflected waves at the input of the antenna and the matching network can be
avoided. This kind of design is not common but eliminates power dissipation in the
matching network.
The Voltage Standing Wave Ratio (VSWR) is a measure of the reflected waves and
the frequency range it is below a prescribed amount defines the bandwidth of the
antenna. Generally the antenna bandwidth is defined as the frequency range over
which the VSWR is less than four. (This indicates that the reflected power is 36% of
the total input power and so, in the absence of resistive losses on the antenna itself,
that 64% of the power is transmitted.)
In addition, the simulations of the antenna e to include the power lost due to the finite
wire conductivity. An investigation of the effect of conductivity on the antenna power
gain and on the radiation pattern should take place. The power gain of the antenna, at
its elevation angle (º = 90º ), should be positive.
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1.4 ORGANISATION OF THIS REPORT
Chapter 1: Introduction (This Chapter)
Chapter 2: State of the Art; contains a historical review and the theoretical
background on general type of antennas based on the published
literature and Web sites of antennas.
Chapter 3: Monopole Antenna; defines common types of monopole antenna and
their advantages and disadvantages. It presents the fundamental
principles of monopole antennas and their applications.
Chapter 4: Designs and Simulations of Loaded Monopole Antenna; contains the
methodology and the design concepts for a broadband monopole. It
describes the wire antenna simulator and displays the results from the
simulations.
Chapter 5: Fabrication and Measurements of Capacitively Loaded Antenna;
accommodates the fabrication and the measurements concepts. It
describes the connectors at the antenna input and displays the results of
the measurements.
Chapter 6: Conclusion; comments on the results of the simulations and
fabrications and concludes with the further work on this project.
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22 SS TT AA TT EE OO FF TT HH EE AA RR TT
2.1 HISTORY OF ELECTROMAGNETISM AND ANTENNAS
The first man, who observed the characteristics of the electron and magnet was the
Greek mathematician, astronomer and philosopher, Thales of Miletus, in 600 BC. He
noticed that when amber is rubbed with silk, it generates sparks and he observed the
generation of attractive forces between two pieces of natural magnetic rock called
“loadstone”. Consequently, the words electron, electricity and their derivatives
derived from the Greek word for amber “ ” (elektron) and the words
magnetism and magnet derived from “ ” (Magnesia), the place in
which the loadstones were found. After the first investigation on electromagnetism,
the first experiment on it took place after 2200 years by the Englishman William
Gilbert, the inventor of the electroscope. Then the American, B. Franklin, defined the
positive and negative charges and the Frenchman, C. A. Coulomb, measured electric
and magnetic forces. In 1831, M. Faraday, in London discovered the production of
electric current from a change in magnetic field [6].
The first radiation experiment took place in Princeton University (1842) by the
inventor of wire telegraphy, Joseph Henry. The initial transmission range of this
experiment was from the upper room to the cellar of Henry’s house, but it was
extended to a distance of over a kilometre. Thomas Edison built the first loaded
antenna in 1885 and it was a top loaded vertical antenna [3].
James Clerk Maxwell set the foundations of antenna design in 1864, with the theory
of electromagnetism. Maxwell died before he could fully exploit his theory, and it
was taken over by Heinrich Hertz. Hertz verified the theory of Maxwell
experimentally in 1887. He built a resonant dipole antenna half-wavelength ( /2)
long, at 400MHz, which was called a “Hertzian dipole”. Hertz also constructed loop
antennas and in 1888 he constructed a parabolic cylinder reflector antenna from a
sheet of zinc [7].
Guglielmo Marconi is the pioneer of radio. He was a wealthy student who was
astounded by the work of Hertz. In 1895, he built a microwave parabolic cylinder
reflector at 1.2GHz. In addition, he discovered that better range could be achieved by
Investigation of Loaded Monopole Antenna
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placing one end of the antenna on the ground while operating at lower frequencies
(<1MHz). According to his observation, Marconi accomplished the first transatlantic
radio communication in 1901 using a 70 kHz spark transmitter connected between the
ground and a system of 50 wires (48-meter fan monopole).
The Second World War saw the launch of the development of radar and the use of
microwave frequencies, which were used in astronomy, satellite and mobile
communications.
At the beginning of the 60’s several studies on loaded antennas and especially on wire
passive loaded antennas took place to achieve broad band characteristics. In 1961,
Altshuler observed that a travelling wave could be maintained along a cylindrical
monopole, if a resistive load of appropriate magnitude (240 ) is set quarter
wavelength ( /4) from the monopole end [8]. B. D. Popovic then explained the
effects of lumped loads on a monopole antenna [9]. Popovic calculated the current
across a cylindrical monopole with lumped load impedance [10]. During the 70’s
extensive investigations on patch or microstrip antennas started, which are popular
for their low profile, their low batch production cost and their specialized geometries.
Nowadays, the investigations of both patch and loaded antennas continue with
astonishing results. M. Bahr, A. Boag, E. Michielssen and R. Mitra designed an ultra
broadband loaded monopole, which operated over the 30-450 MHz band, using
genetic algorithms (GA) to achieve an appropriate result [11][12]. Then K. Yegin and
A. Q. Martin achieved a broadband characteristic in monopole using capacitively
loads [13]. Their initial investigation was extended one year later, achieving very
broadband loaded monopole antenna (bandwidth ratio 64:1) [14].
2.2 M AXWELL EQUATIONS
The Scottish James Clerk Maxwell, a professor at Cambridge University (England),
was the first who identified the electromagnetic waves of any system of conductors
which travel through free space. This work of Maxwell has the title “Treatise on
Electricity and Magnetism” and it was published in 1873 [15].
Maxwell’s equations are generalised equations (laws) of earlier scientists (i.e. Gauss,
Faraday and Ampere).
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The first equation of Maxwell was based on the law for electric and magnetic fields of
a C. F. Gauss (1777-1855), a German mathematician. Gauss’s law associates the total
flux of electric and magnetic fields through a closed surface with the existed electric
charge into this surface. The equations of Gauss have the following form:
Where E is the electric field strength (V/m), is the electrical permittivity of the
medium, is the electric charge density inside the volume V, S is a general closed
surface enclosing the volume V, ds is the vector surface element and B is the magnetic
flux density (Webers/m2 or Tesla).
According to Equation (2.2) the total magnetic flux is always zero, which is verified
by the observation that isolated magnetic charges do not exist in nature.
Maxwell’s equation from Gauss’ law, using the divergence theorem
∫∫ ∫∫∫=S V
dvEdivdsE . (2.3), where E is a vector field and the surface S circles the
volume V, are [16]:
)5.2(0
)4.2(
=
=
Bdiv
Edivε
ρ
The second equation of Maxwell is based on the law of the English physicist M.
Faraday (1791-1867). Faraday’s law defines the electric field, which is generated by
the changes of a magnetic field. It gives the voltage generated in a coil placed in an
alternating magnetic field [17]:
where is the total magnetic flux normal to the coil. Another expression of the same
law that defines the electric field around a closed contour C is equal to the time rate of
change of the total magnetic flux through the contour:
)2.2(0.
)1.2(1
.
∫∫
∫∫ ∫∫∫
=
=
S
S V
dsB
dvdsE ρε
)6.2(t
V∂
Φ∂−=
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)7.2(..∫ ∫∫∂
∂−=
C S
dsBt
dlE
where E is the electric field, C is the closed contour, B is the magnetic field and S the
closed surface. Maxwell using the curl of a vector field such as E-field derived that
[16]:
)8.2(t
BEEcurl
∂
∂−=×∇=
where curl is a derivative with respect to distance in space and is given by:
vectorsunitarezyxwhere
EEE
zyx
zyx
EEcurl
ZYX
)))
)))
,,
∂∂
∂∂
∂∂=×∇=
(2.9)
The last equation of Maxwell is based on the law of the French physicist A. M.
Ampère (1775-1836). Ampere’s law describes the magnetic field set up by a wire
carrying a current:
)10.2(..∫ ∫∫=C S
dsJdlB µ
where C is the closed contour, is the magnetic permeability, S is the closed surface
and J is the conduction current density (Amp/m2). Again Maxwell using the curl of a
vector B derived that [16]:
)11.2(JB µ=×∇
2.3 FUNDAMENTAL OF ANTENNAS
The antenna radiates both electric and magnetic fields in the form of electromagnetic
field. The antenna can be described by lumped elements such as resistance, inductor
and capacitance in order to describe the losses and the radiation efficiency. The
Thevenin equivalent circuit of the antenna as part of a transmitter is displayed below
[18]:
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Figure 2.1 Transmission line Thevenin equivalent circuit of an antenna in transmitter
(Source: [18])
In Figure 2.1 Vg is the generator voltage, Zg is the generator resistance, ZA the antenna
impedance: ZA = (RL + Rr) + jXA, RL is the loss resistance, Rr is the radiation
resistance and XA radiation reactance.
Therefore, the input and the output of the antenna can be represented by a definite
equation of current and voltage. The current produces the magnetic field on the
antenna and the oscillating charge originates the electric field. The polarity of the
antenna and the amount of charge depend on the nature of the output of the
transmitter. The voltage of the antenna, however, depends on the energy source (i.e.
battery, AC/DC source, RF source, etc.). The voltage lags the current when RF source
supplies a half-wavelength ( /2) antenna; hence it acts as a capacitor.
The flow of charge is related to the magnetic field of the antenna and the magnetic
field intensity is proportional to the flow of charge. The current flow is maximum
when the antenna is uncharged, due to the absence of an opposing electric field and
the opposite occurs when the antenna is fully charged.
The finite length of the antenna is the reason for the existence of the standing waves
of the current and the voltage (shown in Figure 2.2). The standing waves are caused
by the incident waves of the RF source when these are halted at the end of the finite
antenna conductor. When the incident waves reach the end of the conductor the
current path is suddenly broken. This interruption is the reason for the collapse of the
magnetic field too. Therefore, the wave, which is reflected back to the input terminals,
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is called a reflected wave. The sum of the incident and reflected waves is the actual
current flow.
Figure 2.2 Standing waves as result of the incident (→) and reflected ( ) waves on
an antenna structure (Source: [19])
The maxima and the minima of the incident and reflected waves are stationary points
respectively. Hence, the resultant wave is a standing wave, due to the stationary points
on the antenna. A standing wave contains fixed minimum points, which are called
nodes and the curves are called loops [19] (shown in Figure 2.3).
Figure 2.3 Loops and nodes of standing waves (Source: [19])
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Reflection Coefficient ( )
The reason for the existence of the standing waves is not only the finite length of the
antenna, but also the impedance match between the antenna and the transmission line.
Generally, the impedance of the antenna ZA is quite different from the characteristic
impedance of transmission line at the input of antenna Zo. The poor match causes the
reflected waves along the transmission line. The reflection coefficient of these waves
is given from the following formula:
i
r
V
V=Γ (2.12)
where Vr is the reflected voltage wave and V i is the incident voltage wave.
Another formula of the reflection coefficient is:
Where ZA is the antenna impedance and Zo is the characteristic impedance of the
transmission line.
The value of during three specific circumstances will be:
• For ZA = Zo ⇒ = 0
• For ZA = 0 ⇒ = -1
• For ZA → ∞ ⇒ = 1
Voltage Standing Wave Ratio (VSWR)
The voltage standing wave is the most important characteristic because its ratio is
used to measure the loss of the antenna due to the reflection waves. The VSWR is
defined as the ratio of the maximum point of the standing wave |V|max (where the
incident wave |V i| is in phase with the reflected one |Vr|) and the minimum point of
)13.2(oA
oA
ZZ
ZZ
+
−=Γ
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 17
the same wave |V|min –(where the incident |V i| and the reflected waves |Vr| are out of
phase 180°).
)14.2(1
1
min
max
Γ−
Γ+=∴
−
+=∴
=
VSWR
VV
VVVSWR
V
VVSWR
ri
ri
The minimum loss happens when the VSWR is equal to one and the maximum when
it tends to infinity.
The value of VSWR during three specific circumstances will be:
• Matched Load: = 0 ⇒VSWR = 1
• Open Circuit: = 1 ⇒ VSWR = ∞
• Short Circuit: = -1 ⇒ VSWR = ∞
The relation of the VSWR with the transmitted power for a mismatched antenna is
given by the following table [20]:
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Table 2.1 VSWR and Transmitted Power for a mismatched Antenna
(Source:[20])
VSWR
REFLECTED POWER (%)
= | |2 X 100
= 1001
12
×
+
−
VSWR
VSWR
TRANSMITTED POWER
(%)
= ( ) 10012
×Γ−
1.0 0.0 100.0
1.5 4.0 96.0
2.0 11.1 88.9
3.0 25.0 75.0
3.5 30.9 69.1
4.0 36.0 64.0
4.5 40.5 59.5
5.0 44.4 55.6
5.83 50.0 50.0
10.0 66.9 33.1
The antenna should have input impedance equal with the characteristic impedance Zo
in order to minimise the losses and the VSWR; this can be achieved by using matching
networks.
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2.4 IMPEDANCE MATCHING AND COUPLING NETWORKS
The fundamental statement in communication systems is that the impedance of the
antenna ZA must be matched to the characteristic impedance of transmission line at the
input of antenna Zo. The transmission line, in turn, has to be matched to the output
impedance of the transmitter. The result of this requirement is to maximise the power
transfer between the source and the load. Antenna impedance ZA contains both
reactive and resistive components usually, but rarely is it a purely resistive impedance
(ZA = R), which is the ideal situation for matching.
As a consequence, the coupling (matching) network consists of lumped, linear, finite,
bilateral components or their distributed counterparts depending on the frequency of
operation. Lumped elements are used, in the lower frequencies (<100MHz), and the
distributed elements are used at higher frequencies. The most common coupling
network is the L- section, shown in Figure 2.4 [21].
Several times an additional element has to be added to form a T or section to retain
the phase constant especially after its change due to matching network, shown in
Figure 2.5.
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 20
(a) (b)
(c)
Figure 2.4 (a) L-section matching network; (b) reverse L-section network; (c) inverted
L-section network (Rs and RL are the source and load impedances respectively)
(Source: [21])
Figure 2.5 Pi ( ) coupling network (Source: [21])
Combination of the above networks can be used to match an antenna to a transmission
line. Another commonly used coupling network is the quarter-wave transformer,
which is used for matching resistances. However, sometimes the requirement for
perfect radiation pattern drives the designer to use a balun, which matches unbalanced
resistive source impedance (i.e. coaxial cable) to a “balanced” load (i.e. antenna) [21].
Rs
C
L
RL
Rs < RL
Rs
RL
C
L
Rs < RL
Rs
L
RLC
Rs > RL
Rs
L
RLC1 C2
Rs > RL
Investigation of Loaded Monopole Antenna
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The above common matching networks are considered for a perfect match at specific
or narrow frequency bandwidth. In other words, bandwidth is not the major design
objective.
The problem of matching starts when a general load has to be matched to a resistance
over a large bandwidth. R. M. Fano, in his report “Theoretical limitations on the
Broadband Matching of Arbitrary Impedances” [22], gave the solution to this
problem. The work of Fano was used for the improvement of antenna matching by R.
L. Tanner [23] [24]. The result of these investigations were analysed by J. Hasik [25]
and he has shown that the maximum bandwidth can be obtained, when the reflection
coefficient ( ) is as constant as possible through the specified bandwidth. After the
designer used this analysis and determined the Q factor of the matching network, he
made it possible to predict the maximum obtainable bandwidth for a given standing
wave ratio (VSWR). The relationship between antenna Q, fractional bandwidth in
terms of geometric mean frequency 21
12
ff
ff −=δ (2.15) and the maximum VSWR for a
matching ladder network of n elements is shown in Figure (2.6).
Figure 2.6 Optimum bandwidth curves for an antenna connected to a network with n
elements (for n=2, n=3 and n=)(Source: [25])
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However, there are antenna structures that can be connected directly to a coaxial line
without matching network, without any effect on system performance. This situation
occurs when the values of the standing wave ratio is small (i.e.< 4) over the specified
bandwidth.
2.5 ANTENNA BANDWIDTH AND W IRELESS COMMUNICATION STANDARDS
During the last two decades new wireless standards have covered many bands of the
frequency spectrum. The most recent wireless communication systems operate within
the 800MHz to 2700MHz range. Therefore the major objective of this project is that
the required monopole operate, at least, over this bandwidth with minimum power
reflection (i.e. VSWR< 4). However, some questions accentuate; why this specified
bandwidth is so important for wireless communication system? Which wireless
standards belong in this bandwidth?
The most common wireless standards, which belong in the above frequency band, are
described below.
2.5.1 Second Generation Cellular Systems (2G)
The major characteristic of the second generation systems is that they are digital and
that they provide voice/data/fax transfer as well as a range of other value-added
services to their users. Nowadays, this system aims at increasing the data rates using
recent technologies such as HSCSD (High Speed Circuit Switched Data) and GPRS
(General Packet Radio Service). Second generation systems (2G) contains several
wireless communication protocols such as the European GSM (Global System for
Mobile Communications), and the American US-TDMA (IS_136), cdmaOne _IS-95)
and the Japanese PDC (Personal Digital Cellular). Both American and Japanese
protocols are based on the preceding first generation (1G) analogue technology.
Unlike these, GSM/IS-95 is based on an exclusively new concept. The major bands of
this system are denoted GSM 900, GSM 1800, and GSM 1900. GSM 900, operating
in the 850 to 950 MHz band, is the most widely used cellular system worldwide
having been adopted in over 100 countries in Europe, Asia, etc. GSM 1800, known
also as PCN or PCN 1800 or DCS 1800, operates in the 1800 to 1900 MHz frequency
Investigation of Loaded Monopole Antenna
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band. This is used in Europe. The third GSM protocol is GSM 1900, known also as
PCS 1900 and DCS 1900, which operates in the 1900 to 2000 MHz frequency band;
This is used primarily in the USA [26].
2.5.2 Digital Enhanced Cordless Telecommunications (DECT)
Another protocol which belongs in the second generation and it is used in the third
also is the DECT. DECT is a European digital wireless technology, which has
expanded to be world-wide. DECT technology is designed for voice and data transfer
over short distances (i.e. less than 300 m, in open area and less than 50 m, when
obstacle exists) [27]. DECT is generated using the best of the existing English
cordless telephony standard CT2 and Swedish standard CT3. The European
Telecommunication Standards Institute (ETSI) developed the DECT standard, which
became reality in 1988. The first ETSI standard for the DECT were published in 1992
(ETS 300 175 and ETS 300 176) and the first product came from Olivetti, in the same
year, which was a wireless LAN-type product [28]. The frequency band of the DECT
according to the ETSI is between 1880-1900 MHz with 10 carriers of 1728 kHz
space. The duplexing technique of DECT is the TDD/TDMA (Time Division
Duplex/Time Division Multiple Access), which uses two different time-slots for
simultaneous transmission and reception. Nowadays, several new DECT wireless
standards are developed such as Extended European DECT (1900-1920 MHz), DECT
China (1900-1920 MHz), DECT South America (1910-1930 MHz) [27].
2.5.3 Bluetooth
Bluetooth is a wireless standard promulgated for wireless connection between many
mobile devices over a short range, enabling users to connect their mobile phones,
laptops, printers and other electronic devices together without using cables [26].
Bluetooth connections are instant without the need for extra settings and all the
devices are in standby condition, searching for compatible devices every 1.28
seconds. The connection is maintained even when devices are not within line of sight,
but they must be in range longer than 10 cm and approximately less than 10 meters,
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but this radio range can be extended to around 100 meters with an optional amplifier
[29]. The maximum number of devices that can be connected in the same group
(piconet) is eight. Bluetooth was generated in February 1998, when five leaders of the
electronics industry, Ericsson, Nokia, IBM, Toshiba, and Intel met to constitute
Bluetooth SIG (Special Interest Group). The first public presentation of this group
took place on the 20th and 21st of May 1998 and in less than a year 500 companies
joined the group. Bluetooth operates on the globally available unlicensed radio band,
2.45 GHz, and supports asynchronous data transfer speeds of up to 721 Kbps, as well
as three voice channels [30]. The advantages of this standard are the small size of the
devices, the low implementation cost and the low power consumption [31].
2.5.4 Third Generation Cellular System (3G)
The third generation systems are developed to unify the existing cellular mobile
systems (2G). The main objective is to offer a wide range of services in different radio
environments with high quality standards. The 3G systems expect to provide high-
speed mobile access to the Internet (based on Internet Protocol (IP)), entertainment,
information and electronic commerce (e-commerce) services. Through IP, the user
can be connected constantly to the Internet to receive his/her e-mail and to retrieve
any information from his/her company network for free. Users will have the
opportunity to set up a videoconference, obtain local tour guides, make a last-minute
reservation at a hotel/restaurant, find and call the nearest taxi firm, or send video
postcards. The high-speed data transfer capability is a result of GPRS (General Packet
Radio Service) and EDGE (Enhanced Data rates for Global Evolution). GPRS will
increase data rates, from 9.6kbit/s to 115kbit/s. Subscribers, using a packet data
service, such as GPRS will always be connected and always on line so services will
be easy and quick to access. EDGE will allow GSM operators to use existing
frequencies to offer wireless multimedia IP-based services and applications at data
rate up to 384 kbit/s or higher. The access to on-line services will be supported by
WAP (Wireless Application Protocol). WAP allows user to access any data
applications from his/her terminal itself, using a built-in browser [32].
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The new mobile terminals for the 3G cellular network will be smaller, “smarter”, they
will contain web browser and e-mail capabilities and they will provide world-wide
roaming. Their screens will be larger to display the videoconferences and the images
captured by the digital camera (shown in Figure 2.7).
Figure 2.7 3G cellular system handheld terminal (Source: [33])
The International Telecommunication Union (ITU) began studies on 3G systems in
the mid-1980s. ITU co-operated with FPLMTS (Future Public Land Mobile
Telecommunication Systems, lately renamed International Mobile
Telecommunications-2000 (IMT-2000) for the development of the 3G cellular
systems.
The European development on 3G cellular systems referred to as Universal Mobile
Telecommunications System (UMTS) is generated by ETSI. UMTS will offer mobile
multimedia services to GSM operators. The major characteristic of UMTS is the
duplex scheme technology that will use Wideband CDMA (WCDMA). This
duplexing standard provides average data transfer speed of 144 kbps, which can be
extended using additional support up to 2Mbps.
ITU has identified the global bands for the 3G systems, which are 1885—2010 MHz
and 2110—2200 MHz for IMT-2000, including the mobile satellite bands of 1980—
2010 MHz and 2170—2200 MHz [34]. In World Radiocommunication Conference
2000, which took place in Istanbul of Turkey in May 2000, ITU (2037 representatives
of 150 countries) identified three new global bands for the 3G systems 806—960
MHz, 1710—1885 MHz and 2500—2690 MHz [35].
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2.5.5 Global Positioning System (GPS)
GPS is a navigation satellite system, which is used to point out different position on
the Earth surface. This system uses satellites in an orbit 20,200 km (12,500 miles)
above the earth and it is more accurate than the older radio navigation systems
because it defines position in three-dimensions using latitude, longitude and altitude.
The Global Positioning System was developed by the U.S. Air Force in 1960. The
system was renamed Navstar, in 1974, when other branches of U.S. military service
joined the research for its development, but the name GPS remained in general usage.
GPS became fully operational in 1995 after a development of $10 billion and twenty-
four satellites circle the Earth every 12 hours to provide global coverage.
In 1972, the accuracy of the system was tested and it was found that the worst case
was 15 m and the best 1 m. The reasons for these wide distances, were the signal time
delay and the ionosphere interference from the satellites to the receiver. Nowadays,
after long research and development and using expensive instruments (accurate
receivers) to avoid the above constraints, the positioning accuracy of GPS has reached
10 mm.
Each satellite has unique P and CA codes, so that the receiver can distinguish between
the transmitters (satellites). Satellite transmits the P code on two signals of different
frequency (L1 = 1575.42 MHz and L2 = 1227.6 MHz). A distinct time delay occurs to
each radio wave every time that the signals pass the ionosphere. However, the
accurate (expensive) GPS receivers are able to track both L1 and L2 and to measure
their difference in arrival. After several calculations, it is easy to determine the delay
caused by the ionosphere [36].
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33 MM OO NN OO PP OO LL EE AA NN TT EE NN NN AA SS
A monopole antenna is theoretically half a dipole, when the ground plane is infinite,
planar and perfectly conducting (i.e. perfect ground). However, it is impossible to
have an infinite plane, even a large ground plane results in a different radiation pattern
from that of an infinite plane. In addition, the capacitance between the base of the
monopole and the ground plane differs from that between two halves of a dipole [37].
This kind of antenna is well known for its compact size, great bandwidth, circular
polarisation, desired impedance level, or particular physical characteristics. When a
monopole is a quarter wavelength long a resonant action occurs and the resonant
resistance is compatible with conventional transmission line feeders. When its
electrical size is less than the quarter of the wavelength then matching and efficiency
problems occur and the feed radiation can ruin the total pattern characteristic [38].
Most of the monopole antennas are “omnidirectional.” It is a cylindrical shaped
antenna, which transmits and receives in 360 degrees. The dimensions of this antenna
typically do not exceed 9cm in diameter and 4.5 m in height. It is also called a "stick
antenna" or "whip antenna" [39].
3.1 FUNDAMENTALS OF MONOPOLE ANTENNA
The linear monopole antenna is half the length of the dipole antenna. However, when
this monopole is mounted on a “perfect” ground plane (i.e. planar, infinite in extent,
and perfectly conducting) the characteristics of the antenna can be derived from dipole
antennas. W. L. Stutzman and G. A. Thiele worked on the perfect ground
characteristics and the image of the antenna’s fields due to the reflection from perfect
ground, which were based on the Snell’s law of reflection. This work is called “Image
Theory” [40]. The following analysis of the monopole characteristics, on a perfect
ground, is also based upon “Image Theory”.
The current (Im) and the charges on a monopole antenna are the same as on the upper
half of a dipole (Idip). The voltage of the monopole (Vm) is half the voltage of the
dipole (Vdip), because the electric field is the same but the length of the antenna is half,
Investigation of Loaded Monopole Antenna
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hence the same electric field over the half distance gives half the voltage. Therefore,
the impedance of the monopole (Zm) antenna is half that of the dipole (Zdip):
The radiation resistance (Rr,m) of the monopole is half that of the dipole (Rr,dip), due to
the monopole radiation power (Pr,m), which emits only over the upper hemisphere.
Hence, it is half of the radiation power of the dipole (Pr,dip) that radiates over a full
sphere.
The directivity (Dm) of the monopole antenna is double the directivity of the dipole
(Ddip). The reason for this is that the radiated power of the monopole is half that of the
dipole for the same current levels. The radiation intensity in the free space for the
monopole ( m) is the same with the dipole one ( dip), due to the unchanged current.
Since there are 4 steradians in the total solid angle the monopole directivity is [41]:
Accordingly from the above equations, the impedance of a quarter-wavelength ( /4)
monopole antenna (Zm,/4 ) is half the impedance of the half-wavelength ( /2) dipole
antenna (Zdip, /2). Thus
)1.3(2
1
21
dipm
dip
dip
m
ZZ
I
VZ
=∴
=
)2.3(2
1,, diprmr RR =
)3.3(2
42
14
dipdip
dip
m
mm D
PPD =
Φ=
Φ=
ππ
Investigation of Loaded Monopole Antenna
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Similarly the directivity of a quarter-wavelength ( /4) monopole antenna (Dm, /4 ) is
double the directivity of the half-wavelength ( /2) dipole antenna (Dd i p , /2) [40].
3.2 TYPES OF MONOPOLE ANTENNA
The number of different types of monopole antennas is limitless. However, the most
characteristic groups of monopole antennas will be described and analysed below.
Linear Monopole
The simplest (structurally) and most widely used antenna is the linear monopole
(shown in Figure 3.1). This type of monopole has radiation resistance (Rr = 40 2
(h/ )2), where h is the height of the antenna from the ground plane and is the
wavelength. The linear monopole is highly capacitive and it has low efficiency when
it is matched due to power losses in the matching network (typically 30-70%) [42].
Figure 3.1 Linear monopole (Source: [42])
( )
)4.3(3.2136
5.42722
1
2
1
4/,
4/,
2/,4/,
Ω+=∴
+=∴
=
jZ
jZ
ZZ
m
m
dipm
λ
λ
λλ
)5.3(16.528.3
64.12
2
4/,
4/,
2/,4/,
dBD
D
DD
m
m
dipm
==∴
×=∴
=
λ
λ
λλ
Investigation of Loaded Monopole Antenna
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Top Loaded Monopole
In this category several wire loaded antennas. These antenna structures increase the
current in the vertical portion of the antenna and this generates an increment of the
radiation resistance (Rr) and a deduction of the input impedance (Zin). The radiation
resistance (Rr) becomes very small and the efficiency of the antennas becomes an
important factor. The radiation resistance (Rr) was computed by Laport, who defined
that the Rr is related to the area ‘A’ (Ampere-Degree) of the plot of current
distribution on the radiating surface. For example when the current at the base of the
monopole is 1 A then the Rr = 0.01215 A2 [37].
“Inverted-L” (shown in Figure 3.2) groups into the top loaded monopole antennas.
The characteristic of this design is the uniform current distribution along the antenna’s
conductor. A consequence of this uniform distribution is to increase the radiation
resistance (Rr). This type of antenna is used at HF and its input impedance is about 5
[42].
Figure 3.2 Inverted-L monopole (Source: [43])
Another type of top loaded monopole is the “Two or Four Element” top loaded
monopole (shown in Figure 3.3). The radiation resistance of these structures is almost
the same as the radiation resistance of the inverted-L monopole. However, these
antennas need tuning and matching networks because they are frequently operated
below self-resonance [42].
Investigation of Loaded Monopole Antenna
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(a) (b)
Figure 3.3 (a) Two/Four-element top loaded monopole (Source: [42])
(b) N-element top loaded monopole (Source:[37])
“Spiral” top loaded monopole (shown in Figure 3.4) belongs, also, this group of
monopoles. It is also self resonant as most top loaded monopoles are. Therefore, it
does not need a matching network for tuning. The efficiency of the “spiral” loaded
monopole is low, about 10%, when it is placed over lossy ground and its height is
around 0.02 . The input impedance of this structure is nearly 6 . It is used at HF
and VLF [42].
Figure 3.4 Spiral top loaded monopole (Source: [42])
One more top loaded monopole the “Capacitor-Plate” monopole (shown in Figure
3.5) is well known for its great radiation resistance. This antenna has a radiation
resistance four times more that of the linear monopole (Rr = 160 2 (h/ )2), where h is
the height of the antenna from the ground plane and is the wavelength [42].
Investigation of Loaded Monopole Antenna
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Figure 3.5 Capacitor plate loaded monopole (Source: [42])
Folded monopole
This monopole is used as a director and reflector in directional antennas. The
radiation resistance of this monopole is around 10–15 . The most common structure
of this antenna is the “Open Folded” monopole (shown in Figure 3.6). The radiation
resistance of this antenna is around 10 . The advantages of using this type of
monopole are the protection of radio equipment from lightning strikes and the ability
to carry shielded cables to a warning light on the top of the antenna without any
important effect on the antenna characteristics [37].
Investigation of Loaded Monopole Antenna
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Figure 3.6 Open folded monopole (Source: [37])
Active/Passive Loaded Monopole
These monopoles are loaded with active (transistor, tunnel diode, varactor, etc.) and
passive (inductor, capacitor, resistance or combinations) elements. The load is used to
increase the radiation resistance, the effective bandwidth and to modify the radiation
pattern of the linear monopole. The performance of the monopole changes according
to the position of the load on the antenna’s conductor [43].
Some examples of this group of monopole antennas are the “Diode Loaded”,
“Transistor Loaded”, “Inductively Loaded” and the “Capacitively Loaded”. The last
structure is analysed in the following chapters of this dissertation.
“Diode Loaded” monopole (shown in Figure 3.7) uses the properties of the varactor
diode. Varying the D.C. biasing of the diode can control the effective height of the
antenna. The structure of the monopole can be reciprocal, when the diode operates
linearly [43].
Investigation of Loaded Monopole Antenna
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Figure 3.7 Diode loaded monopole (Source: [43])
A “Transistor Loaded” antenna (shown in Figure 3.8) is used to reduce the resonance
frequency. Consequently it reduces the effective height of the antenna.
Figure 3.8 Transistor loaded monopole (Source: [43])
The “Inductively Loaded” version (shown in Figure 3.9) increases the efficiency of
the antenna from 50 to 70%. The bandwidth of the same antenna can be increased
nearly 2%, by choosing an appropriate value for the Q factor of the coil. The radiation
resistance of inductively loaded antennas starts from 4 to 23 [42].
Investigation of Loaded Monopole Antenna
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Figure 3.9 Inductively loaded monopole (Source: [42])
Sleeve Monopoles
“Sleeve” monopole (shown in Figure 3.10) is used to increase the radiation resistance
of the antenna and to reduce the height of the antenna. R. A. Burberry has calculated
the input resistance of the sleeve monopole of height l = /4 compared to a linear one
[37]:
)6.3(cos 2 h
RR m
Sβ
=
where RS is the resistance of the sleeve monopole, Rm is the resistance of the linear
monopole, h is the height of the feed-point of the monopole from the ground and is
the phase constant ( = 2 / rad/length).
Other monopole structures, which belong into this group, are “Bent Sleeve”,
“Broadband Sleeve” and “Double-band Sleeve”. These monopoles are used at VHF
and UHF bands [37].
Investigation of Loaded Monopole Antenna
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Figure 3.10 Sleeve monopole (Source: [37])
3.3 ADVANTAGES AND DISADVANTAGES OF MONOPOLE ANTENNAS
Monopole is a special case of a wire antenna. All the other types of wire antennas can
be considered as a general structure of wire antennas in this paragraph.
Monopole antennas are used in several applications that general structure of wire
antennas can be used also. Monopoles have several advantages and disadvantages
compared to general wire antennas. The fundamental advantages and disadvantages of
using monopole antennas compared to wire antennas are displayed below:
Advantages
• Compact size
• Low fabrication cost and simple to manufacture
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• Omni-directional radiation pattern, which allows it to transmit and receive in
360 degrees
• Circular polarisation
• Desired impedance level
• Great bandwidth (using optimum values of loads)
• Resonant antenna
Disadvantages
• Narrow operational bandwidth
• Significant losses in power gain, at high frequencies
The last drawbacks can be minimised by loading the monopole with resonant circuits
(such as capacitors, inductors, diodes, etc.) as happens in the designs of this
dissertation.
3.4 APPLICATIONS OF MONOPOLE ANTENNAS
Monopole antennas are used in several military and civilian applications.
Representative examples are listed in the following table [42], [44]:
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Table3.1 Major Applications of Monopole Antennas
PLATFORM SYSTEMS REASONS
Aircraft Communications, Navigation,
Air Traffic Control
Identification
Low Drag, Low Side Load,
Minimisation of Antenna damage
Vehicles Communication, Navigation Small Electrical Size, Low Height,
Good Omnidirectional Coverage,
Low Drag, Low Weight, Low
Wind and Ice Loading,
Minimisation of Damages due to
Vandals and Environment, Ease of
Replacement, Robust
Marine Communication, Navigation ELF Propagation (submarine),
Small Size, Broadband Coverage
(loaded monopoles),
Omnidirectional Coverage, Low
Water Loading (submarine)
Military Missiles Control, Vehicles
Communication and
Navigation
Good Efficiency, Good
Omnidirectional Coverage,
Broadband Coverage, Robust,
Ease of Replacement and
Concealment
Terminals Mobile Telephony, Radio
Receivers and Navigation
Compact Size, Low Fabrication
Cost, Simple to Manufacture,
Broadband Coverage
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44 DD EE SS II GG NN AA NN DD SSII MM UU LL AA TT II OO NN SS OO FF LL OO AA DD EEDD MMOO NN OO PP OO LL EE
4.1 DESIGN REQUIREMENTS AND CONCEPTS
The most important requirements for a mobile antenna, especially for vehicular use,
are the operating frequency, bandwidth, directivity, pattern characteristics and
polarisation.
It is required that the antenna pattern, for mobile communication, to be 2D
omnidirectional, because the antenna has to transmit and receive in 360 degrees. That
is, the radiation pattern must be circularly symmetrical in the horizontal plane.
Otherwise the propagated signal will vary and it will reduce the communication
quality. In addition, it is preferred that the radiation pattern be maximum in the
horizontal plane (i.e. elevation angle = 90º). Usually the polarisation of the mobile
systems is vertical because it is easier for the designer to model broadband
omnidirectional antennas, such as the monopole of this project. The antenna gain
requirement is to design an antenna with the lowest loss because the power of mobile
devices is limited. The reason for this is the need to battery life of the mobile device
[45].
Generally the most important antenna design requirement is that the antenna achieve a
resonance condition. Nonresonant antennas, such as Vivaldi antennas, use travelling
wave principles and a two or more wavelengths long and so not acceptable in mobile
applications at low microwave frequencies such as one or two gigahertz. The reason is
that the use of a lossy matching network, for tuning and matching, will then be
avoided and the efficiency of the antenna can be kept in high levels.
Apart from the above basic requirements there are several physical, mechanical and
environmental constraints which have to take into consideration in order to choose the
appropriate antenna. The most important of these constraints are [44]:
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 40
Table 4.1 Major Constraints of Vehicular Antenna Design
PHYSICAL MECHANICAL ENVIRONMENTAL
Overhead clearance Low drag Low & high temperature
Hazard to vehicle
passengers
Wind, water & ice loading Humidity
Concealment of vehicle Damage due to vandalism &
cleaning
Rain & ice (resulting in
corrosion)
According to the above requirements and constraints, the optimum type of antenna
that will satisfy them is a monopole. The reasons have been analysed in the previous
chapter.
4.2 DESIGN METHODOLOGY
The initial design of this project was a quarter wavelength ( /4) monopole antenna at
900 MHz. This was an introductory design to observe the characteristics of a simple
monopole with narrow band and determine the capabilities of the simulation software.
The next step was to place a passive load on a monopole structure to increase the
operational bandwidth. The initial approach was based on the method of K. Fujimoto,
A. Henderson, K. Hirasawa and J. R. James [46] for determining the optimum loading
of a monopole antenna. This method makes the input impedance of the monopole real
in order to avoid a lossy matching network but the operational bandwidth was not
satisfactory.
Then different values of passive loads placed along the antenna structure in order the
effective electrical height of the monopole to persist approximately a quarter
wavelength within 900 MHz to 1800 MHz range. Still the results were not
satisfactory.
The design of the loaded monopole continued using the method of K. Yegin and Q.
Martin to model a broadband capacitively loaded monopole [13]. The results of this
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 41
method were satisfactory and they are analysed in this chapter. Then using the
approaches of M. Bahr, A. Boag, et al. [11], [12] the simulations of loaded monopole
continued.
The evaluation of each method was based on the two major requirements:
a) To attain a desired low value for VSWR (i.e. less than four) over the specified
bandwidth.
b) To keep the antenna gain positive.
4.3 DESIGN TOOLS
Simulation of the antennas was accomplished using several tools — the most
significant are analysed in this section
Antenna Model Scaling
“One of the most useful tools of the antenna engineer is the ability to scale his
designs” H. Jasik
According to Maxwell’s linear equations (shown in Chapter 2), an electromagnetic
structure that operates at specified frequency range (f) can have the same properties at
another scaled frequency range (nf). The length of the antenna can be scaled by the
ratio 1/n. The only electromagnetic property that cannot be scaled properly, according
to H. Jasik, is the conductivity of the antenna [25]. Therefore, the next tool that has
been used in this project, is the calculation of the wire conductivity.
Wire Conductivity
The calculation of the wire conductivity requires that the “specific resistance or
resistivity” of the conductor material be determined. The specific resistance of a
material is the resistance of this material with dimensions of one metre in length and
one square metre in cross section and its units are ( m). The most significant
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 42
resistivities of some material, at room temperature, are displayed on the following
table [47], [48]:
Table 4.2 Specific Resistance of some Material of Conductors at Room Temperature
MATERIAL OF CONDUCTOR RESISTIVITY ( M)
Silver 1.6 x 10-8
Copper 1.7 x 10-8
Aluminium 2.8 x 10-8
Brass 7 x 10-8
Iron 10.1 x 10-8
Mercury 96 x 10-8
The resistance of the conductor is given by the following formula [47], [48]:
where R is the resistance of the conductor, S is the specific resistance (resistivity) of
the conductor, L is the length of the conductor, A is the area of cross-section of the
conductor.
The conductance (G) of the material is the reciprocal of its resistance(R):
G = 1/R (Siemens) (4.2)
And the conductivity ( ) of the material is the reciprocal of its resistivity (S):
= 1/S (Siemens/m)
(4.3)
)1.4()(Ω×
=A
LSR
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 43
All of these tools used in collaboration with the most important tool of this project,
the simulation software package for wire antenna structures, NEC.
4.4 NUMERICAL ELECTROMAGNETIC CODE (NEC)
The Numerical Electromagnetic Code (NEC) is a user-oriented computer code, which
is used to analyse the electromagnetic response of antennas and other metal structures.
This code is based on the integral equations of the current that can be generated by the
source or any other incident field. NEC can analyse any antenna model, which
contains transmission lines, perfect or imperfect conductors, lumped elements and the
model can be structured over perfect or imperfect ground. It has been developed at the
Lawrence Livermore Laboratory, Livermore, California, under the sponsorship of the
Naval Ocean Systems Center and the Air Force Weapons Laboratory. It is an
advanced version of the Antenna Modeling Program (AMP), which was produced in
the 1970’s [49].
The antenna structure is modelled with strings of segments following the path of the
wire. The number of segments should be the minimum required for accuracy. The
wire segment is defined by the co-ordinates of its two terminals and its radius. A wire
structure model contains both geometrical and electrical factors. The geometrical
factor is the close sequence of the paths on the conductor and the electrical factor is
the length of the segment (d) relative to the wavelength ( ). A segment length (d) less
than 0.1 and more than 0.001 should be prefered i.e.:
The approximation of the electric field integral equation is based on the Kernel
theory; hence there are two approaches available according to the size of the wire
radius (a):
• The thin-wire Kernel approximation
• The extended thin-wire Kernel approximation
)4.4(1.0001.0 λλ ≤≤ d
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 44
The first approximation should be used when the ratio d/a > 8 for errors less than 1%.
For the same accuracy on the results when the ratio d/a 2 the extended thin-wire
kernel approximation should be used.
For a segment model, some further hints are important [49]:
• Segments should not overlap since the current is indeterminate between two
overlapping segments.
• Analysis accuracy may be reduced when a large radius change takes place
between two connected segments
• At the point at which a voltage source or a network connection is located, a
segment is needed.
• When the voltage source is at the base of a segment connected to a ground
plane, the segment should be vertical.
• The maximum number of wires joined at a single junction is 30 because of a
dimension limitation in the code.
• The segments should be aligned to avoid incorrect current perturbation from
the offset match point and segment junctions, when the wires are parallel and
very close together.
4.4.1 NEC Input Commands (Cards)
The input commands of NEC for executing the antenna structures are called “Cards”.
These cards are divided in three groups, the “Comment Cards”, the “Structure
Geometry Input Cards” and the “Program Control Cards”. The Comment Cards group
contains the NEC commands, which displays the comments of the simulation
program. The second group consists of the cards that defines the antenna geometrical
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 45
structure. The last group includes the NEC cards for the electrical parameters of the
model, the simulation procedures and the data computation parameters.
A brief description of the cards that have been used in this project are displayed below
[49]:
Comment Cards (CM, CE)
A simulation program must begin with a comment card in order to describe the code
that follows. A comment begins with the “CM” card and it is terminated using the
“CE” card.
Wire Specification Card (GW)
This card is one of the Structure Geometry Input Cards. The purpose of this card is to
specify the string of segments required to represent a straight wire. This command is
used to represent the structure (i.e. segments, dimensions) of the simulated monopole.
End Geometry Input Card (GE)
This card is also a Structure Geometry Input Cards. It is used to terminate reading of
geometry data cards and reset the geometry data if a ground plane is following.
Ground Parameters Card (GN)
This is one more card of the third group. Its purpose is to specify the type of ground
used in the simulation. It can define the relative dielectric constant and conductivity of
ground in the proximity of the antenna. Moreover, a second medium (part) of the
ground can be specified using a second set of ground parameters and a radial wire
ground screen can be modeled using a reflection coefficient approximation.
Investigation of Loaded Monopole Antenna
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Frequency Card (FR)
This card belongs into the third group of cards (i.e. Program Control Cards). It is used
to specify the frequency range (in MHz) of the simulation.
Loading Card (LD)
This Program Control Card can be used to specify the impedance loading on one
segment or a number of segments of the antenna structure. There are several options
for defining series and parallel R-L-C loads. One more option is the definition of a
finite conductivity for each segment of the antenna structure.
Excitation Card (EX)
EX card belongs in the third group of cards. Its purpose is to define the antenna
structure excitation. The excitation can be voltage sources on the antenna structure, an
elementary current source, or a plane wave incident on the antenna structure.
Radiation Pattern Card (RP)
RP is a Program Control Card that specifies the sampling parameters for the radiation
pattern calculation and initiates execution of the program.
End of Run Card (EN)
This card also belongs to the third group and is used to indicate the end of all program
execution. It is placed at the end of each program.
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4.5 QUARTER-WAVELENGTH MONOPOLE SIMULATION
According to Carr [19] the Radio Frequency (RF) energy on an antenna moves at
velocity less than the velocity of light (c = 3x108 m/sec). This happens due to the
dielectric constant of the air, which is greater than the dielectric constant of free space
(≈1). The formula of velocity is given by:
Where V is the velocity, f is the frequency and is the wavelength.
Based on this formula, it is clear that velocity of RF energy is proportional to the
wavelength. Hence, a reduction on the velocity will cause a reduction of the physical
length of the antenna too. Therefore, the physical length of the antenna is less than its
electrical length.
The formula for calculating the physical length of an antenna is:
where L(m) is the physical length of the antenna in meters and f(MHz) is the frequency
(in MHz)
Hence, the physical length (in meters) of an antenna at 900 MHz is: L(m) = 0.1585 m
and for a quarter-wavelength monopole L(m) = 0.0792 m.
The range of the segment length (d) for this antenna according to the Equation (4.4)
should be:
0.000333 m < d < 0.0333 m
Consequently, the length of each segment can be 0.0099 m in order to have eight
segments on the antenna structure.
Then the ratio of the segment length (d) to the wire radius (a) becomes:
d/a = 0.0099/0.001 = 9.9 > 8
)5.4(λfV =
)6.4(492
30.0)(
)(
=
MHz
mf
L
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 48
The ratio is greater than eight and according to the NEC-2 manual the thin-wire
Kernel approximation can be used.
Using the above calculations, the initial antenna model for a quarter-wavelength
monopole at 900 MHz (shown in Figure 4.1) was implemented in the NEC simulator.
The input code is given in Appendix A.
Figure 4.1 /4 monopole antenna
VSWR Output Graph
According to the output graph (shown in Figure 4.2), the VSWR for the /4
wavelength is expected to have minimum values at 900 MHz and at 2750 MHz due to
the resonant characteristic of the /4 monopole, at the even multiples of its
operational frequency.
h = /4
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 49
Figure 4.2 VSWR of / 4 monopole at 900 MHz.
The maximum value of VSWR (i.e. maximum reflection) takes place at 1900 MHz,
which is also expected, as long as this frequency is an odd multiple of the resonant
frequency of the /4 monopole. It is clear that the operational bandwidth of the
monopole, according to the design requirements (VSWR < 4), is from 800 MHz to
1100 MHz and from 2550MHz to 3100MHz.
VSWR of Quarter-wavelength Monopole
0
2
4
6
8
10
12
750 950 1150 1350 1550 1750 1950 2150 2350 2550 2750 2950 3150 3350
Frequency (MHz)
VS
WR
VSWR
Investigation of Loaded Monopole Antenna
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Input Impedance Output Graph
The response of a monopole can also be displayed on the Input Impedance graph
(shown in Figure 4.3).
Figure 4.3 Input impedance of /4 monopole at 900 MHz
The reactance of the input impedance at 900 MHz is close to zero and its resistance
tends to 40 , which is close to the radiation resistance of a /4 monopole (36 ).
Hence, most of the input power of the antenna is radiated and just a very small
amount is dissipated due to the loss resistance of the antenna. The input impedance
also is close to the characteristic impedance of the transmission line (Zo = 50 ) and
consequently the standing waves are minimised.
Input Impedance
-400
-300
-200
-100
0
100
200
300
400
500
600
750 950 1150 1350 1550 1750 1950 2150 2350 2550 2750 2950 3150 3350
Frequency (MHz)
Imp
edan
ce (
Oh
ms)
Resistance (Ohms) Reactance (Ohms) Impedance_Mag (Ohms)
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 51
In contrast, at 1900 MHz the impedance has its maximum value due to the maximum
value of its resistance and is far away from the characteristic impedance of the
transmission line.
Current Output Graph
The Current graph (shown in Figure 4.4) displays the magnitude of the current along
the antenna structure (i.e. antenna’s segments) at specific frequencies. This graph
shows that the maximum current obtained at 900 MHz where the input impedance of
the monopole has its minimum value. However, the current magnitude is very low at
1800 MHz, where the VSWR reaches the highest level.
Figure 4.4 Current magnitude along antenna’s segments at specific frequencies
Currents Magnitude vs Antenna Segments
0.00E+00
5.00E-03
1.00E-02
1.50E-02
2.00E-02
2.50E-02
3.00E-02
1 2 3 4 5 6 7 8
Segment
Cu
rren
t (A
mp
)
Current_Magn (900MHz) Current_Magn (1800MHz) Current_Magn (2750 MHz)
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 52
The graph of the current phase (shown in Figure 4.5) along the antenna structure and
at the same frequencies displays the rapid phase change of the current at 2750 MHz,
while at the other frequencies the phase changes at a slow rate. The current phase at
2750 MHz takes place at a distance three-quarters of the way along the antenna from
the ground.
Figure 4.5 Current phase along antenna at specific frequencies
E-Field Pattern Output Graph
The graph shown in Figure 4.6 displays the E-field of the monopole along the
elevation angle of the antenna. The Electric field takes its maximum value (0.5 V/m)
at the elevation angle = 90° (i.e. horizontal plane).
Current Phase vs Antenna Segments
-200
-150
-100
-50
0
50
100
150
200
1 2 3 4 5 6 7 8
Segment
Cu
rren
t P
has
e (D
egre
es)
Current Phase (900 MHz) Current Phase (1800 MHz)
Current Phase (2750 MHz)
Investigation of Loaded Monopole Antenna
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Figure 4.6 E-field pattern vs antenna elevation angle ( º)
Power Gain Output Graph
The same happens with radiation pattern power gain of the antenna (shown in Fig.
4.7), which takes the maximum value of 5 dB at the horizontal plane (i.e. elevation
angle = 90°).
E-Field Pattern vs Angle
0.00E+00
1.00E-01
2.00E-01
3.00E-01
4.00E-01
5.00E-01
6.00E-01
-90
-80
-70
-60
-50
-40
-30
-20
-10 0 10 20 30 40 50 60 70 80
Angle (Degrees)
E-F
ield
(V
/m)
E-Field (V/m)
Investigation of Loaded Monopole Antenna
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Figure 4.7 Power gain (dB) vs antenna elevation angle ( º)
4.6 CAPACITIVELY LOADED MONOPOLE ANTENNA
4.6.1 Capacitively Loaded Monopole with 3 mm Radius of Conductor
The design of a capacitively loaded monopole was based on the method of Yegin and
Martin [13]. This method was analysed and exploited using the antenna design tools,
which were analysed at the beginning of this chapter.
The major problem, during the simulations, was the lack of Genetic Algorithm
software, which could be used to obtain the optimum values of load and their
optimum positions along the monopole. Therefore, the author spent much time
changing the values and the positions of the loads manually.
Power Gain (dB)
-20
-15
-10
-5
0
5
10
-90
-80
-70
-60
-50
-40
-30
-20
-10 0 10 20 30 40 50 60 70 80
Angle (Degrees)
Po
wer
Gai
n (
dB
)
Power Gain (dB)
Investigation of Loaded Monopole Antenna
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After a large number of simulations an acceptable result was derived using two
capacitors C1 = 0.537 pF and C2 = 0.245 pF at the specific positions from the base of
the monopole t1 = 0.055 m and t2= 0.105 m, respectively and 3 mm radius of
conductor (shown in Figure 4.8). The total length of the monopole is 0.143 m.
Figure 4.8 Capacitively loaded monopole antenna
The model of the broadband antenna was placed on a “Perfect Ground” and it does
not contain a matching network. Even without a matching network the output files of
the simulation indicates that the Voltage Standing Wave Ratio (VSWR) is less than 4
from 800 MHz to 3650 MHz. The maximum power gain is around 5.0dB over the
required bandwidth.
The input file from the simulation of this antenna is given in Appendix B.
VSWR Output Graph
Based upon the results of the simulation of the broadband monopole, its VSWR (for a
reference characteristic impedance of Zo=50) shown in Figure 4.9 has minimum
value at 950 MHz and at 3600 MHz. The maximum values of VSWR (maximum loss)
occur at the frequencies from 2400 to 2450 MHz. is graph indicates that the value of
VSWR is less than 4 over the required bandwidth. The maximum values of the graph
are close to 3.5, which correspond to a radiation efficiency of 69.1%, (according to the
Table 2.1, in the second chapter of this report). Significantly, this is achieved over a
broad bandwidth without a matching network.
C
t2
t1
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Figure 4.9 VSWR of capacitively loaded monopole (wire radius 3mm & Zo=50 )
A better VSWR result is achieved by referencing the antenna to a transmission line of
characteristic impedance, Zo, of 75 ) is shown in Figure 4.10.
Figure 4.10 VSWR of capacitively loaded monopole (wire radius 3mm & Zo=75 )
VSWR_ldmnbr3c
0
0.5
1
1.5
2
2.5
3
3.5
4
800 1050 1300 1550 1800 2050 2300 2550 2800 3050 3300 3550 3800
Frequency (MHz)
VS
WR
V S W R
V S W R _ Z o = 7 5 o h m s
0
0.5
1
1.5
2
2.5
3
3.5
800
950
1100
1250
1400
1550
1700
1850
2000
2150
2300
2450
2600
2750
2900
3050
3200
3350
3500
3650
3800
3950
F R E Q U E N C Y ( M H z )
VS
WR
V S W R _ Z o = 7 5 o h m s
Investigation of Loaded Monopole Antenna
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Input Impedance Output Graph
The characteristics of the above antenna are displayed in Figure 4.11 as the input
impedance. The input reactance is close to zero over most of the required bandwidth.
Thus the antenna is self-resonant across its operational bandwidth. In contrast, at 2450
MHz the impedance has its maximum value because of the maximum value of its
resistance (175 ) and far away from the 50 , which is the characteristic impedance
of the transmission line. Similarly the reactance tends to its maximum negative value
(-75) , so the VSWR at this frequency takes its maximum value.
Figure 4.11 Input impedance of capacitively loaded monopole (wire radius 3mm)
Input Impedance of Capacitively Loaded Monopole
-100
-75
-50
-25
0
25
50
75
100
125
150
175
200
800 1050 1300 1550 1800 2050 2300 2550 2800 3050 3300 3550 3800
Frequency (MHz)
Imp
edan
ce (
Oh
ms)
Resistance Reactance Impedance Magn
Investigation of Loaded Monopole Antenna
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Current Output Graph
The currents distribution along the antenna is shown in Figure 4.12 at specific
frequencies. This figure shows that the maximum current is obtained at 800MHz
where the input impedance of the monopole has its minimum value and is close to the
characteristic impedance of the transmission line. Likewise, the current at 3600 MHz
takes large values over the operational bandwidth and its VSWR reaches the
minimum value (VSWR = 1.07). Besides, the currents at the frequencies, where the
VSWR is large (f = 2400 MHz), are very small.
Figure 4.12 Current magnitude along the monopole at specific frequencies
The graph of the current phase, (shown in Figure 4.13) along the antenna structure
and at the same frequencies, displays the rapid phase change of the current at the
Currents Magnitude vs Antenna Segments
0
0.005
0.01
0.015
0.02
0.025
1 2 3 4 5 6 7 8 9 10 11 12 13 14
SEGMENT
CU
RR
EN
T (
Am
p)
Curr_Magn (800MHz) Curr_Magn (950MHz) Curr_Magn (1500MHz) Curr_Magn (1850 MHz)
Curr_Magn (2400 MHz) Curr_Magn (3150 MHz) Curr_Magn (3600 MHz)
Investigation of Loaded Monopole Antenna
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frequencies above the 1850 MHz. The current phase, at the frequencies below the
1850 MHz changes slowly.
Figure 4.13 Currents phase along the monopole at specific frequencies
E-Field Pattern Output Graph
The E-field Pattern at 800 MHz is shown in Fig. 4.14 and its normalised output
confirm the omni-directional characteristic of the monopole antenna. The electric field
takes its maximum value (1.2 V/m) at the elevation angle = 90° (horizontal plane).
Currents Phase vs Antenna Segments
-200
-150
-100
-50
0
50
100
150
200
1 2 3 4 5 6 7 8 9 10 11 12 13 14
SEGMENTS
CU
RR
EN
T P
HA
SE
(D
egre
es)
Curr_Phase (800MHz) Curr_Phase (950MHz) Curr_Phase (1500MHz) Curr_Phase (1850MHz)
Curr_Phase (2400MHz) Curr_Phase (3150MHz) Curr_Phase (3600MHz)
Investigation of Loaded Monopole Antenna
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Figure 4.14 E-Field pattern of the capacitively monopole (wire radius = 3mm)
Power Gain Output Graph
The same happens with the radiation pattern power gain of the antenna, shown in
Figure 4.15, which takes the maximum value of around 5.3 dB at 800 MHz and at the
horizontal place (elevation angle = 90°). The maximum value of the power gain
over the operational bandwidth of the antenna can exceed the 5.3 dB but at different
elevation angle each time. On the other hand, the power gain at the same elevation
angle can become negative at some points of the operational bandwidth.
E-field vs Elevation Angle
0
0.2
0.4
0.6
0.8
1
1.2
1.4
-90
-75
-60
-45
-30
-15 0 15 30 45 60 75 90 -8
0-6
5-5
0-3
5-2
0 -5 10 25 40 55 70
Angle Theta (degrees)
E-f
ield
(V
/m)
E-field
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Figure 4.15 Radiatted power gain as a function of elevation angle
4.6.2 Capacitively Loaded Monopole with 4 mm Radius of Conductor
This design is based on the same method as the previous simulation the only
difference being the radius of the antenna conductor, which is increased here from 1
mm to 4 mm. The length of the antenna and the positions of the two capacitors (as
shown in Figure 4.8) C1 = 0.537 pF and C2 = 0.245 pF remain constant; and t1 = 0.055
m and t2 = 0.105 m, respectively.
The antenna structure is placed on a “Perfect Ground” and does not contain a
matching network. The results of this simulation are much better than the results of
the previous antenna structure with radius of 3 mm.
The input file from the simulation of this monopole is given in Appendix C.
Power Gain vs Elevation Angle
-20
-15
-10
-5
0
5
10
-90 -65 -40 -15 10 35 60 85 -75 -50 -25 0 25 50 75
Angle (degrees)
Po
wer
Gai
n (
dB
)
power gain
Investigation of Loaded Monopole Antenna
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VSWR Output Graph
The VSWR of the monopole (shown in Figure 4.16) connected to transmission line
with characteristic impedance of Zo=50, has minimum value at 950MHz and at
3600MHz. The maximum values of VSWR (maximum loss) are taking place at the
frequencies from 2400 and 3150MHz. However, the most important outcome of this
graph is that the value of VSWR is less than 3.5 over the required bandwidth. The
maximum values of the graph are close to 3.0, which correspond to a radiation
efficiency of more than 75%, (according to the Table 2.1, in the second chapter of this
report). This is a much greater achievement than the previous design.
Fig. 4.16 VSWR of capacitively loaded monopole (wire radius 4mm & Zo=50 )
A better VSWR output achieved by connecting the antenna with a transmission line of
characteristic impedance Zo = 75 (shown in Figure 4.17). The maximum value of
the graph is approximately 2.85 but the most values in the operational bandwidth are
close to 2.00.
VSWR vs Frequency
0
0.5
1
1.5
2
2.5
3
3.5
800
950
1100
1250
1400
1550
1700
1850
2000
2150
2300
2450
2600
2750
2900
3050
3200
3350
3500
3650
3800
3950
FREQUENCY (MHz)
VS
WR
VSWR
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Figure 4.17 VSWR of capacitively loaded monopole (wire radius 4mm & Zo=75 )
Input Impedance Output Graph
The certification of the above results comes from the Input Impedance graph (shown
in Figure 4.18). The susceptance of the input impedance is close to zero and the
impedance magnitude is very close to the antenna resistance over the most of the
required bandwidth. Therefore, the antenna becomes self-resonant along its
operational bandwidth. In contrast, at 2300 MHz the impedance has its maximum
value because of the maximum value of its resistance (145 ) and far away from the
50 , which is the characteristic impedance of the transmission line. Similarly the
susceptance tends to (-43) . The VSWR reaches its maximum value at 3150MHz
where the input reactance has its maximum value (-69 ).
VSWR (Zo=75Ohms)
0
0.5
1
1.5
2
2.5
3
3.5
800
950
1100
1250
1400
1550
1700
1850
2000
2150
2300
2450
2600
2750
2900
3050
3200
3350
3500
3650
3800
3950
FREQUENCY (MHz)
VS
WR
VSWR_75Ohms
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4.18 Input impedance of capacitively loaded monopole (wire radius 4mm)
Current Output Graph
The current magnitude graph, Fig. 4.19, shows that the maximum current is obtained
at 800 MHz where the input impedance of the monopole has its minimum value and
close to the characteristic impedance of the transmission line. Characteristic of this
graph is the fluctuation of the current at 3600 MHz, where the VSWR reaches the
minimum value (VSWR = 1.07). Besides, the currents at the frequencies, where the
VSWR is large (f = 2400 MHz), are very small, as in the previous simulation.
Input Impedance (r = 4mm)
-100
-50
0
50
100
150
200
800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000
FREQUENCY (MHz)
INP
UT
IMP
ED
AN
CE
(O
hm
s)
Input Resistance Input Reactance Imp_ Magnitude
Investigation of Loaded Monopole Antenna
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Figure 4.19 Current magnitude along the monopole at specific frequencies
The graph of the current phase, (shown in Figure 4.20) along the antenna structure
and at the same frequencies, displays the rapid phase change of the current at the
frequencies above the 1850 MHz. The current phase, at frequencies below 1850 MHz
changes slowly.
Currents Magnitude
0
0.005
0.01
0.015
0.02
0.025
1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4
Antenna Segment
Cu
rren
t (A
mp
)
Curr_Magn (800MHz) Curr_Magn (950MHz) Curr_Magn (1500MHz) Curr_Magn (1850 MHz)
Curr_Magn (2400MHz) Curr_Magn (3150MHz) Curr_Magn (3600MHz)
Investigation of Loaded Monopole Antenna
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Figure 4.20 Currents phase along the monopole at specific frequencies
E-Field Pattern Output Graph
The E-field Pattern at 800 MHz, shown in Figure 4.21, and its normalised output
confirm the omni-directional characteristic of the monopole antenna. The electric field
takes its maximum value (1.25 V/m) at the elevation angle = 90° (horizontal plane).
Currents Phase vs Antenna Segment
-200
-150
-100
-50
0
50
100
150
200
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Antenna Segment
Cu
rren
t P
has
e (D
egre
es)
Curr_Phase (800MHz) Curr_Phase (950MHz) Curr_Phase (1500MHz) Curr_Phase (1850 MHz)
Curr_Phase (2400MHz) Curr_Phase (3150MHz) Curr_Phase (3600MHz)
Investigation of Loaded Monopole Antenna
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Figure 4.21 E-field pattern of the capacitively-loaded monopole (wire radius = 4mm)
Power Gain Output Graph
The same happens with the radiation pattern power gain of the antenna at 800 MHz,
shown in Figure 4.22, which takes the maximum value of around 5.37 dB at the
horizontal place (elevation angle = 90°). The maximum value of the power gain
over the operational bandwidth of the antenna can exceed the 5.37 dB but at different
elevation angle each time. On the other hand, the power gain at the same elevation
angle can become negative at some points of the operational bandwidth.
E-Field vs Elevation Angle
0.00E+00
2.00E-01
4.00E-01
6.00E-01
8.00E-01
1.00E+00
1.20E+00
1.40E+00
-90
-70
-50
-30
-10 10 30 50 70 90 -75
-55
-35
-15 5
25 45 65
Elevation Angle (Degrees)
E-F
ield
(V
/m)
E-Field
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Figure 4.22 Radiatted power gain as a function of elevation angle
4.7 LOADED MONOPOLE WITH L-C CIRCUITS
This antenna design is a monopole loaded by parallel L-C circuits, is based on a
“Perfect Ground”, and without a matching network (shown in Fig. 4.23).
Figure 4.23 L-C loaded monopole
T o t a l P o w e r G a i n ( d B )
- 2 0
- 1 5
- 1 0
- 5
0
5
1 0
-90
-65
-40
-15 10 35 60 85 -7
5-5
0-2
5 0 25 50 75
E l e v a t i o n A n g l e ( D e g r e e s )
Po
wer
Gai
n (
dB
)
P o w e r G a i n
L C
tn
t1
Investigation of Loaded Monopole Antenna
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The total length of the monopole is h = 6.6 cm and the four LC circuits are located at
t1 = 0.012 m, t2 = 0.014 m, t3 = 0.016 m and t4 = 0.018 m above the base of the
monopole.
The values of the inductors and capacitors of the loaded circuits along the monopole
is given in the following table:
Table 4.3 Components Values of L-C Circuits
L-C CIRCUITS AT INDUCTOR (L) CAPACITOR (C)
t1 1.5nH 0.45pF
t2 1.5nH 0.40pF
t3 1.5nH 0.35pF
t4 1.5nH 0.30pF
This design is based on the method of Bahr, Boag, Michielssen and Mittra [11], [12].
This method was analysed and it was exploited using the antenna design tools
described at the beginning of this chapter.
The values of the components and their locations along the antenna structure were
obtained after manual iteration. In the centre of the antenna structure, a resistance of
240 was placed to reduce the impedance variation. The VSWR was effected by this
variation, but using the resistance, became flat. The authors of this method mentioned
in [12] that they used a 400 resistance at a height 1/3 below the top of the antenna
to reduce this variation. Even so, in a design without a matching network this value of
resistance did not work as desired and the value suggested by Altshuler [8] was
chosen. The only drawback of incorporating the resistance was the sacrifice of the
antenna radiated power gain.
Investigation of Loaded Monopole Antenna
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The results of these simulations were not as good as for the previous simulations.
However, this is not significant because the use of an optimization procedure such as
the “genetic algorithm” could provide better values for the components and their
locations in order to derive better results. In addition, this method of antenna design
probably needs a matching network that was not used in this simulation. The best
results of this simulation are presented below.
The input file from the simulation of this monopole is given in Appendix D.
VSWR Output Graph
The VSWR graph of the LC loaded monopole is shown in Figure 4.24 with reference
to a transmission line with characteristic impedance, Zo, of 50 , has a minimum
value at 1100 MHz. The maximum values of VSWR are taking place at the upper end
of the simulation band (i.e. above 2900 MHz). It is clear that the fluctuations of the
graph are limited, due to the resistance that is used. However, mostly the VSWR
exceeds 4 and hence the antenna does not satisfy the project.
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Figure 4.24 VSWR of LC loaded monopole
V S W R ( Z o = 5 0 o h m s )
0
1
2
3
4
5
6
7
800
950
1100
1250
1400
1550
1700
1850
2000
2150
2300
2450
2600
2750
2900
3050
3200
F R E Q U E N C Y ( M H z )
VS
WR
V S W R
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Input Impedance Output Graph
The graph of Figure 4.25 presents the input impedance of the LC loaded monopole.
The reactance of the input impedance is not close to zero; hence the impedance
magnitude differs to the antenna resistance over most of the required bandwidth.
Therefore, the antenna is not self-resonant thus its VSWR reaches high levels.
Figure 4.25 Input impedance of L-C loaded monopole
Input Impedance
-200
-150
-100
-50
0
50
100
150
200
250
300
800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200
FREQUENCY (MHz)
INP
UT
IMP
ED
AN
CE
(o
hm
s)
Inp_Resistance Inp_Reactance Inp_Impedance_Mag
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Power Gain Output Graph
The radiation pattern power gain of the antenna (shown in Figure 4.26) reaches
negative values due to the resistance in the centre of antenna. The peak value of the
power is approximately –3.0 dB along the horizontal place (elevation angle = 90°).
However, this negative value does not satisfy the requirement of the project.
Figure 4.26 Radiatted power gain as a function of elevation angle
T o tal Power_Gain
-25
-20
-15
-10
-5
0
-90
-70
-50
-30
-10 10 30 50 70 90 -7
5-5
5-3
5-1
5 5 25 45 65
Angle (Degrees)
Po
wer
Gai
n (
dB
)
Power_Gain
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4.8 SUMMARY
In conclusion, based on the output figures of the above antenna models, it is clear that
the power losses are minimised when the input impedance of the antenna (ZA) is close
to the characteristic impedance (Zo) of the transmission line connected to it and used
as a reference impedance. Additionally, when the reactance XA of the antenna is close
to zero, the input impedance (Zin) of the antenna approximates its resistance (RA). As
a result, the antenna becomes self-resonant and its VSWR gets low values.
Moreover, the current has large value at the frequencies where ZA is low. Therefore,
the impedance of the antenna (ZA) has to be close to Zo in order to minimise losses on
the antenna and to increase its efficiency. At the same time, the current reaches a peak
value, but along the loads of the antenna, it should be close to zero in order to
minimise the losses of the monopole.
The power gain is sacrificed by placing a resistance along the antenna structure.
Therefore, resistance should be avoided in antenna design, where the major objective
is the maximisation of its power gain.
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N. H. VARDALAHOS 08/09/2000Page 75
55 FF AA BB RR II CC AA TT II OO NN && TT EE SS TT II NN GG OO FF CC AA PP AA CC IITT II VV EE LL YY LLOO AA DD EE DD
MM OO NN OO PP OO LL EE
According to the above simulations, satisfactory results were obtained in the
simulation of the capacitively loaded antennas of three and four millimetre conductor
radius. Therefore, the project continues with the fabrication and testing of the
capacitively loaded antenna with a brass conductor of 3mm radius.
5.1 FABRICATION PROCEDURES AND CONCEPTS
The fabrication procedure for the capacitively loaded monopole is analysed below.
• The first step of the fabrication was the manufacturing of several brass
conductor rods of 3 mm radius, in different lengths. The lengths of rods that
were used for the fabrication of the prototypes are displayed on the following
table:
Table 5.1 Lengths of monopole rods
Number of Rods Length of Rods (mm)
2 50
2 45
2 40
1 35
1 30
2 5
2 3
The turning of the rods took place in the workshop of mechanical and electrical
engineering departments at the University of Leeds. The conductors of the
monopole are shown in the Figure 5.1 and Figure 5.2.
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 76
Figure 5.1 Brass rods of the monopole (The scale is in centimetres.)
Figure 5.2 Brass rods of 40mm, 45mm, 5mm and 3mm (The scale is in
centimetres.)
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• The next step in fabricating for the monopole structure was the cutting of the
nylon screws, which were to be used to connect the pieces of brass. Each nylon
screw connects two different rods together, possibly leaving a small gap
between them to simulate a capacitor. This assembly is shown in Figure 5.3.
Figure 5.3 Gap between rods (Simulation of Capacitor)
The length of the gap, which simulates the capacitor, was calculated using the
following formula for parallel-plate capacitor:
Where C is the capacitance of capacitor, is the dielectric of permittivity ( 8.854
x 10–12), A is the area of the capacitor plates (i.e. A = r2, for the cylindrical rods)
and d is the distance between the two plates (i.e. gap between the brass rods).
)1.5(d
AC
ε=
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In order to keep the distance between the two rods stable during antenna testing, a
specific number of films of 1 mm thickness were used, according to the above
calculation of d (shown in Figure 5.4).
Figure 5.4 Prototype capacitively loaded monopole (the films keep the gaps stable)
At the end of the fabrication procedures, the antenna was placed on an “imperfect
ground” and it was connected to the network analyser, using an N-type connector and
an SMA adapter, in order to start with the measurements.
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5.2 FINITE GROUND PLANE (“IMPERFECT GROUND”)
According to Nelson [51], who analysed the method of Meier and Summers on
ground plane design [52], a square ground plane with a side dimension equal to the
diameter of a circular ground plane introduces a small error of the antenna input
impedance. A finite square ground plane is suggested for a monopole antenna design,
as it is easier to be constructed than a circular ground but has comparable error.
Nelson in his analysis of ground plane design, suggested that the correct dimensions
of a finite ground are not the same for all antenna designs. Therefore, the designer
should take into account all the factors, which effect the ground plane of a specific
antenna. A suggested approach is the use of Meier and Summers’ experimental values
[52]. A satisfactory finite ground plane for a monopole with h/a = 150, where (h) is
the length of the monopole and (a) is the radius of its cylindrical conductor, is 6
wavelengths square at its resonance frequency. This ground plane provides an error of
3% at both resonance and antiresonance and an ohmic error of 20 at antiresonance
[51].
In this project the height of the monopole was h = 0.143 m and the radius of its
conductor was a = 0.001 m, so that h/a = 143. The wavelength at 800MHz is =
0.375 m, according to the above theory, the sides of the ground plane should be
approximately 6 , hence they should be 2.25m each. However, it was not feasible to
construct so large a ground plane, because of the limited laboratory space and the lack
of a large aluminium sheet in the departmental workshop.
Thus the square side of the ground plane, which was used in this project, was just
0.35m (shown in Figure 5.5). It is clear that the dimensions of that square ground
were much smaller than the acceptable values determined using the above theory.
Consequently, the author could not expect the result of the testing to be the same as
those of simulations.
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N. H. VARDALAHOS 08/09/2000Page 80
Figure 5.5 Capacitively loaded monopole on a finite square ground plane
5.3 N-TYPE AND SMA CONNECTORS
N-Type Coaxial Connectors
The N-type connector is one of the most popular coaxial connector for microwave
applications. There are male and female N-type connectors. In this project a female
connector was used to connect the antenna to an SMA adapter and then to the network
analyser.
A pair of N-type connectors for 50 ohms systems is shown on the following figure
[50].
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N. H. VARDALAHOS 08/09/2000Page 81
Figure 5.6 Male and female N-type connectors and their dimensions (Source: [50])
This type of connectors has very small reflection and its Voltage Standing Wave Ratio
(SWR) does not exceed 1.07 between DC and 18GHz [50]. The design of N-type
connectors has been improved to prevent damages at their inner parts due to careless
connection. Therefore, during connection of the female with the male connector, the
surface A (shown in Figure 5.6) seats on surface A’ and the surface B touches the
surface B’.
However, coaxial connectors have to always be gently screwed together.
SMA Coaxial Connectors
This type of coaxial connectors is used in low-power microwave applications. The
maximum operational frequency of an SMA connector is approximately 20 GHz. An
SMA connector, like an N-type connector, has small reflection and its SWR is given
by the following formula [50]:
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SWRmax 1.05 + 0.005f GHz (5.2)
Where f is the operational frequency (in GHz)
The design of SMA connectors has been developed to protect their inner parts from
an inattentive use, as was incorporated in N-type connector design. Therefore, the
surface A of the male connector touches the surface A’ of the female and the surface
B stops on the surface B’, during connection. (This is shown in Figure 5.7).
Figure 5.7 Male and female SMA connectors and their dimensions (Source: [50])
The loaded monopole and its connectors are displayed on the Figure 5.8.
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Figure 5.8 Loaded monopole and N-type connector with SMA adapter, under the
ground plane
5.4 ANTENNA MEASUREMENT CONCEPTS
The International Electrotechnical Commission (IEC) has established standards for
the methods of antenna measurement. These regulations are accepted and applicable
world-wide.
IEC has set three stages of measurement for an antenna design, which are [53]:
• Measurement during development
• Measurement during the testing of design
• Measurement during production testing
The major parameters of the antenna measurements according to IEC are:
• Impedance and VSWR measurements
• Gain Measurement
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• Efficiency Measurement
• Radiation Pattern Measurement
These parameters were taken into consideration during the development of the loaded
monopoles of this project. However, all of them could not take place during the
testing procedures, due to the lack of some equipment for antenna measurements.
Therefore during the testing of the prototypes only the Impedance and the VSWR
measurements were performed.
5.5 ANTENNA TESTING AND RESULTS
During the testing of the capacitively loaded monopoles in this project, nineteen
prototypes of different lengths were tested. (An assembled capacitively loaded
monopole is shown in Figure 5.9).
Figure 5.9 Loaded monopole of total height 137.04mm
The prototype, which has been analysed in Section 4.6.1 of this report, produced the
best outcomes, during the testing. The characteristics of this antenna are given in the
following table.
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Table 5.2: Characteristics of the tested monopole
Height from the
Ground
(cm)
Values of
Capacitor
(pF)
Gap Distance (d) between
Antenna’s Rods
(Simulating Capacitors)
(mm)
Load 1 5.5 0.537 0.467
Load 2 10.5 0.245 1.02
Total length of Loaded Monopole: 14.3cm
Radius of antenna Conductor (Brass): 3.0mm
Loaded monopoles of 4 mm radius were not been tested due to the lack of 4 mm brass
conductors.
Testing Procedures
The procedures that were followed to test the above capacitively loaded monopole are
as follows.
• The first step was the calibration of the Network Analyser and its fixturing. A
reference plane was established at the input of the antenna. The calibration of
the network analyser was done using the components of Hewlett Packard
(shown in Fig. 5.10).
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Figure 5.10 HP components for the calibration of network analyser
• The test frequency range was from 500MHz to 4,000MHz and the resolution
was 201 points (i.e. frequencies) between the frequency range.
• The next step was the measurement of the reflection parameter (S11) and
VSWR of the antenna prototypes. During this procedure the author was
changing the length of the monopole and the height of the loads from the
antenna base. As was mentioned above, the best results were derived for the
capacitively loaded monopole of Table 5.2.
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VSWR Measurement Output
The VSWR derived from the measurements of the capacitively loaded monopole,
specified in Table 5.2, is displayed in the following figure.
Figure 5.11 VSWR measurement of capacitively loaded antenna
Input Impedance Measurement Output
The graph of the input impedance (shown in Fig. 5.12) was derived from the antenna
reflection parameter (S11) using the MATLAB program. (The program listing is
given in APPENDIX E). The network analyser measures the antenna impedance at
the end of its cable. Thus, the MATLAB program is used to calculate the impedance
at the input of the monopole. MATLAB code computes the phase shift of the
impedance via the transmission line that exists between the cable of network analyser
and the antenna input. In order to determine the input impedance, an assumption that
V S W R M E A S U R E M E N T
0
0 .5
1
1 .5
2
2 .5
3
3 .5
4
4 .5
5
5 .5
7 6 1 . 1 9 5 1 1 9 6 . 5 2 1 6 3 1 . 8 4 5 2 0 6 7 . 1 7 2 5 0 2 . 4 9 5 2 9 3 7 . 8 2 3 3 7 3 . 1 4 5 3 8 0 8 . 4 7
F R E Q U E N C Y ( M H z )
VS
WR
V S W R
Investigation of Loaded Monopole Antenna
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the transmission line is lossless, took place. Therefore, the phase shift of the
impedance was computed by using the following formula:
Where is the phase shift, is the propagation constant and l is the length of the
transmission line.
Figure 5.12 Input impedance measurement of loaded monopole
)3.5(lβφ =
500 1000 1500 2000 2500 3000 3500 4000-100
-50
0
50
100
150
200Input Impedance of Loaded Monopole
Frequency (MHz)
Impe
danc
e (O
hms)
Magnitude
ResistanceReactance
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5.6 SUMMARY
According to the graphs of VSWR and input impedance it is clear that the results of
the testing are not the same as those simulated. Nevertheless, it was expected that
there would be a difference. This difference is a result of the finite (“imperfect”)
ground that was used as a base of the antenna during the testing. As was mentioned in
Section 5.2, the dimensions of the ground were not satisfactory. Another measurement
constraint was the laboratory area (shown in Figure 5.13) where the testing took place.
The reflections of metal elements in the laboratory environment affected the antenna
performance.
Figure 5.13 Laboratory environment
In addition, the rods of the antenna were not exactly in a vertical direction due to the
existence of slightly bending of the nylon screws. One more constraint was the films
between the two rods. These films were used to keep the distance between the two
rods fixed (i.e. the capacitor plates stable). These discontinuities create “fringing”
fields, which store additional energy from the signal transmission along the antenna
structure. This storage is operating as an additional capacitor.
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66 CC OO NN CC LL UU SS II OO NN SS
Mobile telephony is one of the major communication tools in our lives. It is increasing
rapidly with mobile phones rapidly becoming important accessories and their users
being around 100 millions all over the world at the end of 1999. On the other hand,
new standards of wireless telecommunication transferred from the research centres in
short time intervals, which make the need for broadband equipment mandatory. Thus,
companies are endeavouring to develop broadband devices such as the first dual-band
vehicular mobile handsets, which was presented during the international exhibition of
mobile telephony, CEBIT 2000.
The major objective of this project was the construction of a broadband monopole
antenna for vehicular use, which can be operated with the latest protocols of wireless
communication. In particular the objective in this study was minimisation of the
antenna losses, including that of a matching network if present, over the operational
broad bandwidth. The bandwidth was taken to be the frequency range over which the
VSWR was less than four and the antenna power gain positive.
The monopole antenna has interesting characteristics such as its omni-directional
radiation pattern and its easy construction. However, at high frequencies its
operational bandwidth becomes narrow and the losses affecting power gain are
significant. Loading the monopole with reactive elements (such as capacitors,
inductors, etc.) can reduce these disadvantages.
The last statement is verified by considering the outputs of the simulation of a loaded
broadband monopole. According to the simulation presented here the broad
operational bandwidth of the loaded monopole (800 to 4,000 MHz) enables it to be
used with several protocols such as GSM (900MHz/1800MHz/1900MHz), DECT
(1900 MHz), Bluetooth (2.45GHz), UMTS and GPS. This operational bandwidth
corresponds to a ratio of 11:1 achieved using a 0.143 m capacitively-loaded
monopole.
The VSWR of the capacitively loaded monopole with a conductor radius of 4 mm,
over the operational bandwidth and without matching network, was close to three.
This indicates that the reflected power is 25% of the total input power and so, in the
absence of resistive losses on the antenna itself, it further indicates that 75% of the
power is transmitted. This efficiency is much better than the efficiency of previous
Investigation of Loaded Monopole Antenna
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designs, especially without using matching network and it denotes a great
achievement.
Based on the simulation outputs of the loaded monopoles, it is clear that the power
losses are minimised when the input impedance of the antenna (ZA) is close to the
characteristic impedance (Zo) of the transmission line which is connected to it.
Additionally the antenna becomes self-resonant and its VSWR yields low values
when the reactance (XA) of the antenna is close to zero, and the input impedance (Zin)
of the antenna approximates its resistance (RA).
Moreover, the current has a large value at the frequencies where ZA is low, but along
the antenna loads it should be close to zero in order to improve the antenna efficiency.
Resistances should be avoided in antenna design, where the major objective is the
minimisation of losses, because they sacrifice antenna power gain.
According to the fabrication of the capacitively loaded monopole of 3mm conductor
radius, the outcomes of testing did not agree very closely with the simulated results.
However the broadband response was demonstrated. The major reason for the
variation between the simulation and measured input impedance was attributed to the
finite ground. The ground that was used during the measurements was much smaller
than the appropriate size for approximating an infinite ground, hence a phase shift in
the input impedance was generated. With a small ground plane the antenna radiation
pattern has several large sidelobes. The dimensions of a square ground plane should
be approximately 200 (estimation) to obtain an almost perfect monopole radiation
[37]. Moreover the laboratory environment was not the best for testing an antenna
because the reflections of neighbouring metal elements reduced the antenna
performance.
Further reasons for this variation were the slight bending resulting from the use of
nylon screws (required as they are non-conducting). The nylon screws could not keep
the rods of the antenna in a precise vertical orientation.
The achievements of the project were satisfactory; all the deliverables of proposal
were achieved. Extra deliverables took place and the whole project was finished
according to plan.
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 92
6.1 RECOMMENDATION FOR FURTHER W ORK
The following section contains a number of suggestions for further work which might
be carried out in order to further elucidate what factors govern the design,
performance and fabrication of the prototype loaded monopole and how these can be
manipulated. The Project fulfils the design requirements and it has achieved all the
objectives. However, the following suggestions would improve the functionality of
the loaded antenna.
First of all a capacitively-loaded antenna with 4 mm conductor radius should be
manufactured and tested in order to compare its results with those of the 3 mm wire
radius. According to the simulations of these antennas the conductor of 4 mm radius
provides better results and broader bandwidth that the wire of 3 mm radius.
The testing of the existing capacitively loaded monopoles should be continued over a
broader bandwidth. A number of simulations over the frequency range from 800MHz
to12GHz gave acceptable results but there was not enough time to test it.
A very important tool that could improve the existing designs is a Genetic Algorithm
(GA). This code could achieve better values and locations for the loads of the antenna
in order to improve its characteristics over broadband operation. GA could be used to
obtain values for the elements of a matching network, if the latter is necessary in a
broadband antenna design.
The last but not the least suggestion for the continuation of the project is the
conversion of the existing prototype designs to a patch microstrip antenna design. The
patch antenna is constructed on a microstrip using printed circuit fabrication
techniques and the microstrip layer guides the antenna radiation. Such an antenna with
the characteristics of the existing prototypes could be used in any handheld terminal
or even in base stations for broadband transmission and reception.
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 93
77 RR EE FF EE RR EE NN CC EE SS
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Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 98
APPENDIX A
APPENDIX A
Simulation Code
of /4 Monopole Antenna at 900 MHz
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 99
CM This is an Input file of a Monopole Antenna with radius 0.001m
CM and length 0.079m(divided in 8 segments)at 900MHz and
CM excitation source 1V at base, over a perfect ground.
CM Wire (Brass) Conductivity: 14285714.3 S/m
CE
GW 1,8, 0,0,0, 0,0,0.079, 0.001
GE 1
GN 1
FR 0 55 0 0 700 50
LD 5 1 1 8 14285714.3
EX 0 1 1 00 1.0 0.0
RP 0 36 73 1000 -90 0 5 5
EN
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APPENDIX B
APPENDIX B
Simulation Code
of a Capacitively Loaded Antenna
(with radius a = 3 mm)
Investigation of Loaded Monopole Antenna
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CM This is the Input file of a Loaded Broadband Monopole Antenna
CM with radius 0.003m and length 0.143m
CM Capacitive Loaded monopole at L1=0.055m and at L2=0.105m
CM with C1=0.537pF and C2=0.245pF respectively.
CM Conductivity of Brass 14285714.3 S/m is included.
CM Excitation source 1V at base, over a perfect ground.
CE
GW 1,14, 0,0,0, 0,0,0.143, 0.003
GE 1
GN 1
FR 0 65 0 0 800 50
LD 5 1 1 14 14285714.3
LD 0 1 6 6 0 0 0.537E-12
LD 0 1 11 11 0 0 0.245E-12
EX 0 1 1 00 1.0 0.0
RP 0 72 72 1000 -90 0 5 5
EN
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APPENDIX C
APPENDIX C
Simulation Code
of a Capacitively Loaded Antenna
(with radius a = 4 mm)
Investigation of Loaded Monopole Antenna
N. H. VARDALAHOS 08/09/2000Page 103
CM This is the Input file of a Loaded Broadband Monopole Antenna
CM with radius 0.004m and length 0.143m
CM Capacitive Loaded monopole at L1=0.055m and at L2=0.105m
CM with C1=0.537pF and C2=0.245pF respectively.
CM Conductivity of Brass 14285714.3 S/m is included.
CM Excitation source 1V at base, over a perfect ground.
CE
GW 1,14, 0,0,0, 0,0,0.143, 0.004
GE 1
GN 1
FR 0 65 0 0 800 50
LD 5 1 1 14 14285714.3
LD 0 1 6 6 0 0 0.537E-12
LD 0 1 11 11 0 0 0.245E-12
EX 0 1 1 00 1.0 0.0
RP 0 72 72 1000 -90 0 5 5
EN
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APPENDIX D
APPENDIX D
Simulation Code
of a L-C Loaded Antenna
Investigation of Loaded Monopole Antenna
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CM This is the Input file of a Loaded Broadband Monopole Antenna
CM with radius 0.001m and length 0.066m
CM Parallel L-C Loaded monopole at l1=0.012m, l2=0.014m, l3=0.016m
CM and l4=0.018m with C1=0.45pF, L1=L2=L3=L4=1.5nH C2=0.398pF,
CM C3=0.353pF
CM and C4= 0.293pF respectively. Resistance 240 Ohms at 3.3cm
CM Conductivity of Brass 14285714.3 S/m
CM Excitation source 1V at base, over a perfect ground.
CE
GW 1,66, 0,0,0, 0,0,0.066, 0.001
GE 1
GN 1
EK 0
LD 1 1 12 12 0 1.5E-9 0.45E-12
LD 1 1 14 14 0 1.5E-9 0.398E-12
LD 1 1 16 16 0 1.5E-9 0.353E-12
LD 1 1 17 17 0 1.5E-9 0.293E-12
LD 0 1 33 33 240
LD 5 1 1 66 14285714.3
FR 0 50 0 0 800 50
EX 0 1 1 00 1.0 0.0
RP 0 72 72 1000 -90 0 5 5
EN
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APPENDIX E
APPENDIX E
MATLAB Code
for the Input Impedance Measurement
of Capacitively Loaded Antenna
(with radius a = 3 mm)
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clear;clc;
Zo=50;
load s11reim21.txt; %Load file of S11 real & imaginary arraymatrix
freq = s11reim21(:,1); %Set the frequency ranges11re = s11reim21(:,2); %Set the values of S11 real partss11im = s11reim21(:,3); %Set the values of S11 imaginary parts
lamda = 3*10^2./freq; %Set the wavelength beta = 2*pi./lamda; %Set the propagation constant
S11 = (s11re)+j*(s11im);%Reflection Parameter S11
l = 0.05; %Distance between Antenna Input andNetwork Analyser Cable
phi = l.*beta; %Electrical Length of this Distance
Gamma = S11.*exp(-j*2.*phi)%Set Reflection Coefficient Gammag = abs(Gamma); %Magnitude of Reflection Coeff. Gammarg = angle(Gamma); %Phase of Reflection Coeff. theta = -2.*phi+Gammarg;
Zin =Zo.*(1+Gamma)./(1-Gamma); %Input Impedance Zinmag = abs(Zin); %Magnitude of the Input Impedance Zinre = real(Zin); %Input Resistance Zinim = imag(Zin); %Input Reactance
vswr = (1+Gammag)./(1-Gammag) %VSWR at the antenna Input
figure(1)plot(freq,Zinmag,'b',freq,Zinre,'g',freq,Zinim,'r');title('Input Impedance of Loaded Monopole');xlabel('Frequency (MHz)');ylabel('Impedance (Ohms)');grid;
figure(2)plot(freq,vswr);title('SWR of Loaded Monopole');xlabel('Frequency (MHz)');ylabel('VSWR');grid;
figure(3)polar(phi,Gammag);title('Reflection Coeff. of Loaded Monopole');
figure(4)polar(theta,Gammag);title('Reflection Coeff. of Loaded Monopole (Input)');